MDEV-25606: Concurrent CREATE TRIGGER statements mix up in binlog and break replication

The bug is that we don't have a a lock on the trigger name, so it is
possible for two threads to try to create the same trigger at the same
time and both thinks that they have succeed.
Same thing can happen with drop trigger or a combinations of create and
drop trigger.

Fixed by adding a mdl lock for the trigger name for the duration of the
create/drop.

sql/sql_trigger.cc

@@ -475,7 +475,7 @@ bool mysql_create_or_drop_trigger(THD *thd, TABLE_LIST *tables, bool create)
   }
 
   /* Protect against concurrent create/drop */
-  MDL_REQUEST_INIT(&mdl_request_for_trn, MDL_key::TABLE,
+  MDL_REQUEST_INIT(&mdl_request_for_trn, MDL_key::TRIGGER,
                    create ? tables->db.str : thd->lex->spname->m_db.str,
                    thd->lex->spname->m_name.str,
                    MDL_EXCLUSIVE, MDL_EXPLICIT);