Process commands in command queues.

def command_queue_worker(self, command_queue):
        """Process commands in command queues.
        """
        while True:
            try:
                # set timeout to ensure self.stopping is checked periodically
                command, data = command_queue.get(timeout=3)
                try:
                    self.process_command(command, data)
                except Exception as e:
                    _logger.exception(e)
                    self.worker_exceptions.append(e)
                    break
            except Empty:
                pass
            if self.stopping and (_worker_fast_exit_on_terminate or command_queue.empty()):
                break