function _waitForCursor(ctx, cb) {
  if (ctx.cursor) {
    return cb();
  }
  ctx.once('cursor', function() {
    cb();
  });
}
Adds a [cursor flag](https://mongodb.github.io/node-mongodb-native/4.9/classes/AggregationCursor.html#addCursorFlag).
Useful for setting the `noCursorTimeout` and `tailable` flags.

@param {String} flag
@param {Boolean} value
@return {AggregationCursor} this
@api public
@method addCursorFlag