HTTP/2: optimized processing of small DATA frames.

HTTP/2: optimized processing of small DATA frames.

The request body filter chain is no longer called after processing
a DATA frame.  Instead, we now post a read event to do this.  This
ensures that multiple small DATA frames read during the same event loop
iteration are coalesced together, resulting in much faster processing.

Since rb->buf can now contain unprocessed data, window update is no
longer sent in ngx_http_v2_state_read_data() in case of flow control
being used due to filter buffering.  Instead, window will be updated
by ngx_http_v2_read_client_request_body_handler() in the posted read
event.