MDEV-23080: desync and pause node on BACKUP STAGE BLOCK_DDL

make BACKUP STAGE behave as FTWRL, desyncing and pausing the node
to prevent BF threads (appliers) from interfering with blocking stages.
This is needed because BF threads don't respect BACKUP MDL locks.

Reviewed-by: Jan Lindström <jan.lindstrom@mariadb.com>

sql/sql_class.h

@@ -3011,6 +3011,9 @@ public:
   uint	     server_status,open_options;
   enum enum_thread_type system_thread;
   enum backup_stages current_backup_stage;
+#ifdef WITH_WSREP
+  bool wsrep_desynced_backup_stage;
+#endif /* WITH_WSREP */
   /*
     Current or next transaction isolation level.
     When a connection is established, the value is taken from