_worker(task, callback) {
    const context = task.owner;
    debug('processing batch (%s items)', task.data.length);
    context.throttle.submit(context._sendBatch, context, task.data, callback);
  }
Send a batch of contacts to Twilio SendGrid.

@param {Object} task Task to be processed (data in 'data' property)
@param {Function} callback Callback function.