Return new stream ID from nghttp2_submit_{request, headers, push_promise}

Previously stream ID was assigned just before HEADERS or PUSH_PROMISE
was serialized and nghttp2_submit_{request, headers, push_promise} did
not return stream ID.  The application has to check assigned stream ID
using before_frame_send_callback.  Now it is apparent that priority is
meant to DATA transfer only.  Also application can reorder the
requests if it wants. Therefore we can assign stream ID in
nghttp2_submit_* functions and return stream ID from them.  With this
change, now application does not have to check stream ID using
before_frame_send_callback and its code will be simplified.

lib/nghttp2_frame.h

@@ -436,6 +436,7 @@ int nghttp2_frame_pack_blocked(nghttp2_bufs *bufs, nghttp2_blocked *frame);
  */
 void nghttp2_frame_headers_init(nghttp2_headers *frame,
                                 uint8_t flags, int32_t stream_id,
+                                nghttp2_headers_category cat,
                                 const nghttp2_priority_spec *pri_spec,
                                 nghttp2_nv *nva, size_t nvlen);