begin method
Implementation
TxnContextV2 begin({bool readOnly = false}) {
final txnId = _nextTxnId++;
final activeSnapshot = Set<int>.from(_active.keys);
final ctx = TxnContextV2(
txnId: txnId,
snapshotTxnId: _lastCommitted,
activeAtBegin: activeSnapshot,
readOnly: readOnly,
);
_active[txnId] = ctx;
return ctx;
}