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/sql_reload.cc

@@ -308,8 +308,8 @@ bool reload_acl_and_cache(THD *thd, unsigned long long options,
             with global read lock.
           */
           if (thd->open_tables &&
-              !thd->mdl_context.is_lock_owner(MDL_key::GLOBAL, "", "",
-                                              MDL_INTENTION_EXCLUSIVE))
+              !thd->mdl_context.is_lock_owner(MDL_key::BACKUP, "", "",
+                                              MDL_BACKUP_STMT))
           {
             my_error(ER_TABLE_NOT_LOCKED_FOR_WRITE, MYF(0),
                      thd->open_tables->s->table_name.str);