Close the connection/shutdown the messaging loop.
        :param payload: None: not used. Here to allow using this method with add_command.

def shutdown(self, payload=None):
        """
        Close the connection/shutdown the messaging loop.
        :param payload: None: not used. Here to allow using this method with add_command.
        """
        logging.info("Work queue shutdown.")
        self.connection.close()
        self.receiving_messages = False