Start a thread to dispatch requests queued up by callbacks.
        Spawns a thread to run :meth:`dispatch_callback`.

def start(self):
        """Start a thread to dispatch requests queued up by callbacks.
        Spawns a thread to run :meth:`dispatch_callback`.
        """
        with self._operational_lock:
            if self._thread is not None:
                raise ValueError("Dispatcher is already running.")

            flow_control = self._manager.flow_control
            worker = helper_threads.QueueCallbackWorker(
                self._queue,
                self.dispatch_callback,
                max_items=flow_control.max_request_batch_size,
                max_latency=flow_control.max_request_batch_latency,
            )
            # Create and start the helper thread.
            thread = threading.Thread(name=_CALLBACK_WORKER_NAME, target=worker)
            thread.daemon = True
            thread.start()
            _LOGGER.debug("Started helper thread %s", thread.name)
            self._thread = thread