Patch by Ian Babrou, with minor changes.

Patch by Nick Marden, with minor changes.

And corresponding variable $connections_waiting was added.

Previously, waiting connections were counted as the difference between
active connections and the sum of reading and writing connections.
That made it impossible to count more than one request in one connection
as reading or writing (as is the case for SPDY).

Also, we no longer count connections in handshake state as waiting.

In r2411 setting of NGX_HTTP_GZIP_BUFFERED in c->buffered was moved from
ngx_http_gzip_filter_deflate_start() to ngx_http_gzip_filter_buffer() since
it was always called first.  But in r2543 the "postpone_gzipping" directive
was introduced, and if postponed gzipping is disabled (the default setting),
ngx_http_gzip_filter_buffer() is not called at all.

We must always set NGX_HTTP_GZIP_BUFFERED after the start of compression
since there is always a trailer that is buffered.

There are no known cases when it leads to any problem with current code.
But we already had troubles in upcoming SPDY implementation.

Not only this is useful for the upcoming SPDY support, but it can
also help to improve HTTPS performance by enabling TLS False Start
in Chrome/Chromium browsers [1].  So, we always enable NPN for HTTPS
if it is supported by OpenSSL.

[1] http://www.imperialviolet.org/2012/04/11/falsestart.html