Web Socket Session
Flush all outstanding messages and suspend until all earlier sent messages will be written. Could be called at any time even after close. May return immediately if the connection is already terminated. However it may also fail with an exception (or cancellation) at any point due to session failure. Please note that flush doesn't guarantee that frames were actually delivered.
Enqueue frame, may suspend if outgoing queue is full. May throw an exception if outgoing channel is already closed so it is impossible to transfer any message. Frames that were sent after close frame could be silently ignored. Please note that close frame could be sent automatically in reply to a peer close frame unless it is raw websocket session.