docs: 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;
  }