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.test

@@ -32,7 +32,7 @@ connection con2;
 --echo # Wait until COMMIT gets blocked.
 let $wait_condition=
   select count(*) = 1 from information_schema.processlist
-  where state = "Waiting for commit lock" and info = "COMMIT";
+  where state = "Waiting for backup lock" and info = "COMMIT";
 --source include/wait_condition.inc
 --echo # Verify that 'con1' was blocked and data did not move.
 SELECT * FROM t1;