message: work around race condition in doctest for drain
diff: @@ -1248,6 +1248,8 @@ impl Connection {
  /// nc.publish("test.drain", "message")?;
  /// nc.drain()?;
  ///
  + /// # std::thread::sleep(std::time::Duration::from_secs(1));
  + ///
  /// assert!(received.load(SeqCst));
  ///
  /// # Ok(())
  