Returns the local (receive) window size for a connection. The local window size can be adjusted by
nghttp2_submit_window_update(). This function takes into account that and returns effective window size.
This function does not take into account the amount of received data from the remote endpoint. Use
nghttp2_session_get_local_window_size()to know the amount of data the remote endpoint can send without receiving connection-level WINDOW_UPDATE frame. Note that each stream is still subject to the stream level flow control.
This function returns -1 if it fails.