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.

--no-location-rewrite option disallows location header rewrite on
--http2-bridge, --client and default mode.  This option is useful when
connecting nghttpx proxy with --http2-bridge to backend nghttpx with
http2-proxy mode.

It might be useful to clean the unused stream out to make up the room
for new streams.  On the other hand, proxy should maintain the
connection between upstream client and downstream server and they have
the timeout for their own.  Proxy just reacts to their decision.