title: Acknowledgment for socket.io custom event
body: <p>I am looking for a method to acknowledge a socket.emit call.</p>
  <pre><code>socket.emit('message', msg);
  </code></pre>
  <p>I have seen a mechanism where the receiver would send another custom event as an acknowledgement, but this would add thousands of transports in my chat application. Please advice an efficient method.</p>