function callback (error, res) {
    if (self.execFn) {
      return self.execFn(error, res, _callback);
    } else {
      return _callback(error, res);
    }
  }
Get all matching elements
Will return pointers to matched elements (shallow copies), returning full copies is the role of find or findOne
This is an internal function, use exec which uses the executor

@param {Function} callback - Signature: err, results