beginTransaction method
Implementation
Future<int> beginTransaction(int txnId, int tableId) async {
_currentTxnId = txnId;
return append((lsn, prev) => WalBinRecord(
lsn: lsn, txnId: txnId, prevLsn: prev,
op: WalBinOp.begin, flags: 0, tableId: tableId, rowId: 0,
));
}