Delete all data of a queue
@param {String} qName Queue name
@returns {Promise} A promise when all data of the queue is deleted
function(qName) {
    if (!qName || qName.length === 0) {
      throw new Error('You must specify a queue name.');
    }
    var client = redis.client();
    return client.keysAsync('bull:' + qName + ':*').then(function(keys) {
      if (keys.length) {
        return client.del(keys);
      }
    });
  }