func (db *DB) NewTransactionAt(readTs uint64, update bool) *Txn {
	if !db.opt.managedTxns {
		panic("Cannot use NewTransactionAt with managedDB=false. Use NewTransaction instead.")
	}
	txn := db.newTransaction(update, true)
	txn.readTs = readTs
	return txn
}
func (db *DB) NewTransactionAt(readTs uint64, update bool) *Txn {
	if !db.opt.managedTxns {
		panic("Cannot use NewTransactionAt with managedDB=false. Use NewTransaction instead.")
	}
	txn := db.newTransaction(update, true)
	txn.readTs = readTs
	return txn
}
// NewTransactionAt follows the same logic as DB.NewTransaction(), but uses the
// provided read timestamp.
//
// This is only useful for databases built on top of Badger (like Dgraph), and
// can be ignored by most users.