message: Flush message sender when a potentially awaited message is being sent
diff: @@ -690,10 +690,12 @@ pub(super) async fn message_sender<P>(
  };
  limiter.request_allowance(estimated_wire_size).await;
  - let outcome = sink.send(message).await;
  + let mut outcome = sink.send(message).await;
  // Notify via responder that the message has been buffered by the kernel.
  if let Some(responder) = opt_responder {
  + // Since someone is interested in the message, flush the socket to ensure it was sent.
  + outcome = outcome.and(sink.flush().await);
  responder.respond(()).await;
  }
  