Combine GLOBAL and COMMIT namespaces into BACKUP namespace.

Part of MDEV-5336 Implement LOCK FOR BACKUP

Other things:
- Added printing of MDL locks to DBUG.

sql/handler.cc

@@ -1431,8 +1431,7 @@ int ha_commit_trans(THD *thd, bool all)
       We allow the owner of FTWRL to COMMIT; we assume that it knows
       what it does.
     */
-    mdl_request.init(MDL_key::COMMIT, "", "", MDL_INTENTION_EXCLUSIVE,
-                     MDL_EXPLICIT);
+    mdl_request.init(MDL_key::BACKUP, "", "", MDL_BACKUP_COMMIT, MDL_EXPLICIT);
 
     if (!WSREP(thd) &&
       thd->mdl_context.acquire_lock(&mdl_request,