introduced noop in message callbacks.

This is just a cosmetic change so the callback in message could be avoided:

````js
// Get a message
q.get(function(err, message) {
  // do something with the message
  // then delete it
  message.del();

  // need more time?
  // touch the message
  message.touch();

  // can't process the message
  // release the message
  message.release();
});


````