managed_db.go
func (txn *Txn) CommitAt(commitTs uint64, callback func(error)) error {
	if !txn.db.opt.managedTxns {
		panic("Cannot use CommitAt with managedDB=false. Use Commit instead.")
	}
	txn.commitTs = commitTs
	if callback == nil {
		return txn.Commit()
	}
	txn.CommitWith(callback)
	return nil
}
func (txn *Txn) CommitAt(commitTs uint64, callback func(error)) error {
	if !txn.db.opt.managedTxns {
		panic("Cannot use CommitAt with managedDB=false. Use Commit instead.")
	}
	txn.commitTs = commitTs
	if callback == nil {
		return txn.Commit()
	}
	txn.CommitWith(callback)
	return nil
}
// CommitAt commits the transaction, following the same logic as Commit(), but
// at the given commit timestamp. This will panic if not used with managed transactions.
//
// This is only useful for databases built on top of Badger (like Dgraph), and
// can be ignored by most users.