Previous change 1ce3f01a4355 incorrectly introduced processing of the
ngx_posted_next_events queue at the end of operation, effectively making
posted next events a nop, since at the end of an event loop iteration
the queue is always empty.  Correct approach is to move events to the
ngx_posted_events queue at an iteration start, as it was done previously.

Further, in some cases the c->read event might be already in the
ngx_posted_events queue, and calling ngx_post_event() with the
ngx_posted_next_events queue won't do anything.  To make sure the event
will be correctly placed into the ngx_posted_next_events queue
we now check if it is already posted.

Introduced in 9d2ad2fb4423 available bytes handling in SSL relied
on connection read handler being overwritten to set the ready flag
and the amount of available bytes.  This approach is, however, does
not work properly when connection read handler is changed, for example,
when switching to a next pipelined request, and can result in unexpected
connection timeouts, see here:

http://mailman.nginx.org/pipermail/nginx-devel/2019-December/012825.html

Fix is to introduce ngx_event_process_posted_next() instead, which
will set ready and available regardless of how event handler is set.

Added code to track number of bytes available in the socket.
This makes it possible to avoid looping for a long time while
working with fast enough peer when data are added to the socket buffer
faster than we are able to read and process data.

When kernel does not provide number of bytes available, it is
retrieved using ioctl(FIONREAD) as long as a buffer is filled by
SSL_read().

It is assumed that number of bytes returned by SSL_read() is close
to the number of bytes read from the socket, as we do not use
SSL compression.  But even if it is not true for some reason, this
is not important, as we post an additional reading event anyway.

Note that data can be buffered at SSL layer, and it is not possible
to simply stop reading at some point and wait till the event will
be reported by the kernel again.  This can be only done when there
are no data in SSL buffers, and there is no good way to find out if
it's the case.

Instead of trying to figure out if SSL buffers are empty, this patch
introduces events posted for the next event loop iteration - such
events will be processed only on the next event loop iteration,
after going into the kernel and retrieving additional events.  This
seems to be simple and reliable approach.

In theory, this can provide a bit better distribution of latencies.

Also it simplifies the code, since ngx_queue_t is now used instead
of custom implementation.

It's mostly dead code.  And the idea of thread support for this task has
been deprecated.

    *) Change: the "bl" and "af" parameters of the "listen" directive was
       renamed to the "backlog" and "accept_filter".

    *) Feature: the "rcvbuf" and "sndbuf" parameters of the "listen"
       directive.

    *) Change: the "$msec" log parameter does not require now the
       additional the gettimeofday() system call.

    *) Feature: the -t switch now tests the "listen" directives.

    *) Bugfix: if the invalid address was specified in the "listen"
       directive, then after the -HUP signal nginx left an open socket in
       the CLOSED state.

    *) Bugfix: the mime type may be incorrectly set to default value for
       index file with variable in the name; the bug had appeared in 0.3.0.

    *) Feature: the "timer_resolution" directive.

    *) Feature: the millisecond "$upstream_response_time" log parameter.

    *) Bugfix: a temporary file with client request body now is removed
       just after the response header was transferred to a client.

    *) Bugfix: OpenSSL 0.9.6 compatibility.

    *) Bugfix: the SSL certificate and key file paths could not be relative.

    *) Bugfix: the "ssl_prefer_server_ciphers" directive did not work in
       the ngx_imap_ssl_module.

    *) Bugfix: the "ssl_protocols" directive allowed to specify the single
       protocol only.

    *) Feature: the ngx_http_ssi_module supports "include virtual" command.

    *) Feature: the ngx_http_ssi_module supports the condition command like
       'if expr="$NAME"' and "else" and "endif" commands. Only one nested
       level is supported.

    *) Feature: the ngx_http_ssi_module supports the DATE_LOCAL and
       DATE_GMT variables and "config timefmt" command.

    *) Feature: the "ssi_ignore_recycled_buffers" directive.

    *) Bugfix: the "echo" command did not show the default value for the
       empty QUERY_STRING variable.

    *) Change: the ngx_http_proxy_module was rewritten.

    *) Feature: the "proxy_redirect", "proxy_pass_request_headers",
       "proxy_pass_request_body", and "proxy_method" directives.

    *) Feature: the "proxy_set_header" directive. The "proxy_x_var" was
       canceled and must be replaced with the proxy_set_header directive.

    *) Change: the "proxy_preserve_host" is canceled and must be replaced
       with the "proxy_set_header Host $host" and the "proxy_redirect off"
       directives, the "proxy_set_header Host $host:$proxy_port" directive
       and the appropriate proxy_redirect directives.

    *) Change: the "proxy_set_x_real_ip" is canceled and must be replaced
       with the "proxy_set_header X-Real-IP $remote_addr" directive.

    *) Change: the "proxy_add_x_forwarded_for" is canceled and must be
       replaced with
       the "proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for"
       directive.

    *) Change: the "proxy_set_x_url" is canceled and must be replaced with
       the "proxy_set_header X-URL http://$host:$server_port$request_uri"
       directive.

    *) Feature: the "fastcgi_param" directive.

    *) Change: the "fastcgi_root", "fastcgi_set_var" and "fastcgi_params"
       directive are canceled and must be replaced with the fastcgi_param
       directives.

    *) Feature: the "index" directive can use the variables.

    *) Feature: the "index" directive can be used at http and server levels.

    *) Change: the last index only in the "index" directive can be absolute.

    *) Feature: the "rewrite" directive can use the variables.

    *) Feature: the "internal" directive.

    *) Feature: the CONTENT_LENGTH, CONTENT_TYPE, REMOTE_PORT, SERVER_ADDR,
       SERVER_PORT, SERVER_PROTOCOL, DOCUMENT_ROOT, SERVER_NAME,
       REQUEST_METHOD, REQUEST_URI, and REMOTE_USER variables.

    *) Change: nginx now passes the invalid lines in a client request
       headers or a backend response header.

    *) Bugfix: if the backend did not transfer response for a long time and
       the "send_timeout" was less than "proxy_read_timeout", then nginx
       returned the 408 response.

    *) Bugfix: the segmentation fault was occurred if the backend sent an
       invalid line in response header; the bug had appeared in 0.1.26.

    *) Bugfix: the segmentation fault may occurred in FastCGI fault
       tolerance configuration.

    *) Bugfix: the "expires" directive did not remove the previous
       "Expires" and "Cache-Control" headers.

    *) Bugfix: nginx did not take into account trailing dot in "Host"
       header line.

    *) Bugfix: the ngx_http_auth_module did not work under Linux.

    *) Bugfix: the rewrite directive worked incorrectly, if the arguments
       were in a request.

    *) Bugfix: nginx could not be built on MacOS X.

    *) Bugfix: nginx did run on Linux parisc.

    *) Feature: nginx now does not start under FreeBSD if the sysctl
       kern.ipc.somaxconn value is too big.

    *) Bugfix: if a request was internally redirected by the
       ngx_http_index_module module to the ngx_http_proxy_module or
       ngx_http_fastcgi_module modules, then the index file was not closed
       after request completion.

    *) Feature: the "proxy_pass" can be used in location with regular
       expression.

    *) Feature: the ngx_http_rewrite_filter_module module supports the
       condition like "if ($HTTP_USER_AGENT ~ MSIE)".

    *) Bugfix: nginx started too slow if the large number of addresses and
       text values were used in the "geo" directive.

    *) Change: a variable name must be declared as "$name" in the "geo"
       directive. The previous variant without "$" is still supported, but
       will be removed soon.

    *) Feature: the "%{VARIABLE}v" logging parameter.

    *) Feature: the "set $name value" directive.

    *) Bugfix: gcc 4.0 compatibility.

    *) Feature: the --with-openssl-opt=OPTIONS autoconfiguration directive.

    *) Bugfix: the proxied request was sent without arguments if the
       request contains "//", "/./", "/../" or "%XX".

    *) Bugfix: the large compressed responses may be transferred not
       completely.

    *) Bugfix: the files bigger than 2G was not transferred on Linux that
       does not support sendfile64().

    *) Bugfix: while the build configuration on Linux the
       --with-poll_module parameter was required; the bug had appeared in
       0.1.8.