@abc.abstractmethod<EOL><INDENT>@synchronized(lock)<EOL>def dequeue(self, destination):<DEDENT>
Removes and returns an item from the queue (or C{None} if no items in queue).

@param destination: The queue name (destinationination).
@type destination: C{str}

@return: The first frame in the specified queue, or C{None} if there are none.
@rtype: C{stompclient.frame.Frame}