merge.go
func (op *MergeOperator) Add(val []byte) error {
	return op.db.Update(func(txn *Txn) error {
		return txn.Set(op.key, val)
	})
}
func (op *MergeOperator) Add(val []byte) error {
	return op.db.Update(func(txn *Txn) error {
		return txn.Set(op.key, val)
	})
}
// Add records a value in Badger which will eventually be merged by a background
// routine into the values that were recorded by previous invocations to Add().