public void acknowledge(Message msg) {
        Reply ack = new EmptyReply();
        msg.swapState(ack);
        reply(ack);
    }
Convenience method for acknowledging a message for its sender.

This is equivalent to:
<pre>
    Reply ack = new EmptyReply();
    ack.swapState(msg);
    reply(ack);
</pre>

Messages should be acknowledged when
<ul>
    <li>this destination has safely and permanently applied the message, or
    <li>an intermediate determines that the purpose of the message is fulfilled without forwarding the message.
</ul>

@param msg The message to acknowledge back to the sender.
@see #reply
messagebus/src/main/java/com/yahoo/messagebus/DestinationSession.java