_setupQueue() {
    // Create a queue that wil be used to send batches to the throttler.
    this.queue = queue(ensureAsync(this._worker));

    // When the last batch is removed from the queue, add any incomplete batches.
    this.queue.empty = () => {
      if (!this.pendingItems.length) {
        return;
      }

      debug('adding %s items from deferrd queue for processing', this.pendingItems.length);

      const batch = this.pendingItems.splice(0);
      this._pushToQueue(batch);
    };

    // Emit an event when the queue is drained.
    this.queue.drain = () => {
      this.emit('drain');
    };
  }
Sets up the queue object on this instance of ContactImporter