tdb/begin: use txn id in mem

source/libs/tdb/src/db/tdbDb.c

@@ -103,7 +103,7 @@ int32_t tdbBegin(TDB *pDb, TXN **ppTxn, void *(*xMalloc)(void *, size_t), void (
                  int flags) {
   SPager *pPager;
   int     ret;
-  int64_t txnId = 1;
+  int64_t txnId = ++pDb->txnId;
 
   TXN *pTxn = tdbOsCalloc(1, sizeof(*pTxn));
   if (!pTxn) {

source/libs/tdb/src/inc/tdbInt.h

@@ -382,6 +382,7 @@ struct STDB {
 #ifdef USE_MAINDB
   TTB *pMainDb;
 #endif
+  int64_t txnId;
 };
 
 typedef struct hashset_st *hashset_t;