Revision 1716592

Implement basic windowing support for http/2. For now just use a constant
window for the connection and each stream and update it when it falls under
a treshold.

* protocols/http2_protocol.c
  (serf_http2_protocol_t): Add some vars.
  (http2_send_window_update): New function.
  (serf__http2_protocol_init,
   serf__http2_protocol_init_server): Initialize dynamic window state.
     Use http2_send_window_update instead of hardcoded window update.
  (http2_process): Update window when needed.

* protocols/http2_protocol.h
  (serf_http2_stream_t): Add some vars.

* protocols/http2_stream.c
  (serf_http2__stream_create): Initialize dynamic window state.