Revert "libmdb: Do not use the MDB_txbody.mtb_txnid field."

This reverts commit 4a0b30499c.

libraries/libmdb/mdb.c

@@ -511,17 +511,11 @@ typedef struct MDB_txbody {
 		 */
 	pthread_mutex_t	mtb_mutex;
 #endif
-#if MDB_VERSION == 1
 		/**	The ID of the last transaction committed to the database.
 		 *	This is recorded here only for convenience; the value can always
 		 *	be determined by reading the main database meta pages.
-		 *
-		 *	Value is unused, but maintained for backwards compatibility.
-		 *	Drop this or mtb_me_toggle when changing MDB_VERSION.
-		 *	(Reading both should have been done atomically.)
 		 */
 	txnid_t		mtb_txnid;
-#endif
 		/** The number of slots that have been used in the reader table.
 		 *	This always records the maximum count, it is not decremented
 		 *	when readers release their slots.
@@ -542,9 +536,7 @@ typedef struct MDB_txninfo {
 #define mti_version	mt1.mtb.mtb_version
 #define mti_mutex	mt1.mtb.mtb_mutex
 #define mti_rmname	mt1.mtb.mtb_rmname
-#if MDB_VERSION == 1
 #define mti_txnid	mt1.mtb.mtb_txnid
-#endif
 #define mti_numreaders	mt1.mtb.mtb_numreaders
 #define mti_me_toggle	mt1.mtb.mtb_me_toggle
 		char pad[(sizeof(MDB_txbody)+CACHELINE-1) & ~(CACHELINE-1)];
@@ -1584,19 +1576,18 @@ mdb_txn_renew0(MDB_txn *txn)
 			pthread_setspecific(env->me_txkey, r);
 		}
 		txn->mt_toggle = env->me_txns->mti_me_toggle;
-		txn->mt_txnid = r->mr_txnid = env->me_metas[txn->mt_toggle]->mm_txnid;
-
+		txn->mt_txnid = env->me_txns->mti_txnid;
 		/* This happens if a different process was the
 		 * last writer to the DB.
 		 */
 		if (env->me_wtxnid < txn->mt_txnid)
 			mt_dbflag = DB_STALE;
+		r->mr_txnid = txn->mt_txnid;
 		txn->mt_u.reader = r;
 	} else {
 		LOCK_MUTEX_W(env);
 
-		txn->mt_toggle = env->me_txns->mti_me_toggle;
-		txn->mt_txnid = env->me_metas[txn->mt_toggle]->mm_txnid;
+		txn->mt_txnid = env->me_txns->mti_txnid;
 		if (env->me_wtxnid < txn->mt_txnid)
 			mt_dbflag = DB_STALE;
 		txn->mt_txnid++;
@@ -1604,6 +1595,7 @@ mdb_txn_renew0(MDB_txn *txn)
 		if (txn->mt_txnid == mdb_debug_start)
 			mdb_debug = 1;
 #endif
+		txn->mt_toggle = env->me_txns->mti_me_toggle;
 		txn->mt_u.dirty_list = env->me_dirty_list;
 		txn->mt_u.dirty_list[0].mid = 0;
 		txn->mt_free_pgs = env->me_free_pgs;