If the main request was finalized while a background request performed an
asynchronous operation, the main request ended up in ngx_http_writer() and was
not finalized until a network event or a timeout.  For example, cache
background update with aio enabled made nginx unable to process further client
requests or close the connection, keeping it open until client closes it.

Now regular finalization of the main request is not suspended because of an
asynchronous operation in another request.

If a background request was terminated while an asynchronous operation was in
progress, background request's write event handler was changed to
ngx_http_request_finalizer() and never called again.

Now, whenever a request is terminated while an asynchronous operation is in
progress, connection error flag is set to make further finalizations of any
request with this connection lead to termination.

These issues appeared in 1aeaae6e9446 (not yet released).

The sched_setaffinity() function was introduced in DragonFly BSD 4.7,
so it is no longer Linux-specific.

Prodded by Sepherosa Ziehau.

In http these checks were changed in a6d6d762c554, though mail module
was missed at that time.  Since then, the stream module was introduced
based on mail, using "== NGX_ERROR" check.

With OpenSSL 1.1.0+, the workaround for handshake buffer size as introduced
in a720f0b0e083 (ticket #413) no longer works, as OpenSSL no longer exposes
handshake buffers, see https://github.com/openssl/openssl/commit/2e7dc7cd688.
Moreover, it is no longer possible to adjust handshake buffers at all now.

To avoid additional RTT if handshake uses more than 4k we now set TCP_NODELAY
on SSL connections before handshake.  While this still results in sub-optimal
network utilization due to incomplete packets being sent, it seems to be
better than nothing.

Previously, cache background update might not work as expected, making client
wait for it to complete before receiving the final part of a stale response.
This could happen if the response could not be sent to the client socket in one
filter chain call.

Now background cache update is done in a background subrequest.  This type of
subrequest does not block any other subrequests or the main request.

Previously, the read event of the accepted connection was marked ready, but not
available.  This made EPOLLRDHUP-related code (for example, in ngx_unix_recv())
expect more data from the socket, leading to unexpected behavior.

For example, if SSL, PROXY protocol and deferred accept were enabled on a listen
socket, the client connection was aborted due to unexpected return value of
c->recv().

If allocation of cleanup handler in the HTTP/2 header filter failed, then
a stream might be freed with a HEADERS frame left in the output queue.

Now the HEADERS frame is accounted in the queue before trying to allocate
the cleanup handler.