Motivation:

The send window size is currently fixed by a macro at compile time.
In order for users of the library to impact the send window size they
would have to change a macro at compile time. The window size may be dynamic
depending on the environment and deployment scheme. The library users
currently have no way to change this parameter.

Modifications:

Add a new optional callback method which is called before data is sent to
obtain the desired send window size. The callback return value will be
subject to a range check for the current session, stream, and settings
limits defined by flow control.

Result:
Library users have control over their send sizes.

Previously we only update consumed flow control window when number of
bytes read in nghttp2 and spdylay callback is 0.  Now we notify
nghttp2 library the consumed bytes even if number of bytes read > 0.
This change also uses newly added spdylay_session_consume() API, so we
require spdylay >= 1.3.0.

This is partial revert of bbe4f5a3.
Only documentation is reverted.  Since we have 2 queues to handle
maximum concurrent streams, we are not ready to allow immediate frame
submission for pending new frames.

Android lacks /dev/stderr, so directly use /proc/self/fd/2 as default
errorlog-file.  Android does not like O_APPEND for /proc/self/fd/1 and
/proc/self/fd/2, so omit the flag for these paths.

This commit makes handling of outgoing HEADERS and PUSH_PROMISE in the
same priority of other frames on the stream, so these frames are
processed in the order they are submitted.  This allows application to
submit frames to a stream returned by nghttp2_submit_{request,
headers, push_promise} immediately.  The only exception is
WINDOW_UPDATA frame, which requires nghttp2_stream object, which is
not created yet.

--read-burst=0 still does not work.  But specifying n > 0 workarounds
this.

This option limits the number of backend connections per frontend.
This is meaningful for the combination of HTTP/2 and SPDY frontend and
HTTP/1 backend.

libnghttp2 will call on_stream_close callback when RST_STREAM is
received.  So we can use on_stream_close callback to handle existing
stream, instead of on_frame_recv callback.

nghttpx supports hot deploy feature using signals.  The host deploy in
nghttpx is multi step process.  First send USR2 signal to nghttpx
process.  It will do fork and execute new executable, using same
command-line arguments and environment variables.  At this point, both
current and new processes can accept requests.  To gracefully shutdown
current process, send QUIT signal to current nghttpx process.  When
all existing frontend connections are done, the current process will
exit.  At this point, only new nghttpx process exists and serves
incoming requests.