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.