code: function DbWrkrMongoDB(opt) {
    this.host = opt.host || 'localhost';
    this.port = opt.port || 27017;
    this.dbName = opt.dbName;
  
    assert(this.dbName, 'has database name');
    this.db = null;
    this.dbSubscriptions = null;
    this.dbQitems = null;
  }
docs: Document schema  subscripions: eventName   String  (indexed) queues      [String]  qitems: name        String  (indexed) queue       String  (indexed) tid         String payload     Object parent      _ObjectId created     Date when        Date    (sparse indexed) done        Date    (sparse indexed) retryCount  Number