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.

mysql-test/main/flush_block_commit_notembedded.test

@@ -46,7 +46,7 @@ begin;
 connection con1;
 let $wait_condition=
   select count(*) = 1 from information_schema.processlist
-  where state = "Waiting for global read lock" and
+  where state = "Waiting for backup lock" and
         info = "insert into t1 values (1)";
 --source include/wait_condition.inc
 unlock tables;