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/suite/perfschema/r/stage_mdl_global.result

@@ -22,7 +22,7 @@ call dump_one_thread('user2');
 username	event_name	sql_text
 user2	statement/sql/insert	insert into test.t1 values (1), (2), (3)
 username	event_name	nesting_event_type
-user2	stage/sql/Waiting for global read lock	STATEMENT
+user2	stage/sql/Waiting for backup lock	STATEMENT
 username	event_name	nesting_event_type
 user2	stage/sql/Init	STATEMENT
 user2	stage/sql/Checking permissions	STATEMENT

mysql-test/suite/perfschema/t/stage_mdl_global.test

@@ -9,7 +9,7 @@ flush tables with read lock;
 
 connect (con2, localhost, user2, , );
 
-# Will wait on con1, "Waiting for global read lock"
+# Will wait on con1, "Waiting for backup lock"
 --send
 insert into test.t1 values (1), (2), (3);
 
@@ -26,7 +26,7 @@ let $wait_condition=
 let $wait_condition=
   select count(*) = 1 from performance_schema.threads
   where `TYPE`='FOREGROUND' and PROCESSLIST_USER like 'user2'
-  and PROCESSLIST_STATE = 'Waiting for global read lock';
+  and PROCESSLIST_STATE = 'Waiting for backup lock';
 --source include/wait_condition.inc
 
 call dump_one_thread('user1');