The directives limit the upstream read rate.  For example,
"proxy_limit_rate 42" limits proxy upstream read rate to
42 bytes per second.

GetQueuedCompletionStatus() document on MSDN says the
following signature:

    http://msdn.microsoft.com/en-us/library/windows/desktop/aa364986.aspx

    BOOL WINAPI GetQueuedCompletionStatus(
      _In_   HANDLE CompletionPort,
      _Out_  LPDWORD lpNumberOfBytes,
      _Out_  PULONG_PTR lpCompletionKey,
      _Out_  LPOVERLAPPED *lpOverlapped,
      _In_   DWORD dwMilliseconds
    );

In the latest specification, the type of the third argument
(lpCompletionKey) is PULONG_PTR not LPDWORD.

This prevents inappropriate session reuse in unrelated server{}
blocks, while preserving ability to restore sessions on other servers
when using TLS Session Tickets.

Additionally, session context is now set even if there is no session cache
configured.  This is needed as it's also used for TLS Session Tickets.

Thanks to Antoine Delignat-Lavaud and Piotr Sikora.

The new directives {proxy,fastcgi,scgi,uwsgi,memcached}_next_upstream_tries
and {proxy,fastcgi,scgi,uwsgi,memcached}_next_upstream_timeout limit
the number of upstreams tried and the maximum time spent for these tries
when searching for a valid upstream.

Some of the OpenSSL forks (read: BoringSSL) started removing unused,
no longer necessary and/or not really working bug workarounds along
with the SSL options and defines for them.

Instead of fixing nginx build after each removal, be proactive
and guard use of all SSL options for bug workarounds.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>

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.