tdb2: move transaction lock

Make it the same offset as TDB1.  This isn't strictly necessary, but
it would allow for total unification later, since TDB1 and TDB2's
transaction code is otherwise completely compatible.

ccan/tdb2/private.h

@@ -77,10 +77,10 @@ typedef int tdb_bool_err;
 
 /* Prevent others from opening the file. */
 #define TDB_OPEN_LOCK 0
-/* Doing a transaction. */
-#define TDB_TRANSACTION_LOCK 1
 /* Expanding file. */
 #define TDB_EXPANSION_LOCK 2
+/* Doing a transaction. */
+#define TDB_TRANSACTION_LOCK 8
 /* Hash chain locks. */
 #define TDB_HASH_LOCK_START 64