MDEV-30780 optimistic parallel slave hangs after hit an error

The hang could be seen as show slave status displaying an error like
    Last_Error: Could not execute Write_rows_v1
along with
    Slave_SQL_Running: Yes

accompanied with one of the replication threads in show-processlist
characteristically having status like

   2394 | system user  |    | NULL | Slave_worker | 50852| closing tables

It turns out that closing tables worker got entrapped in endless looping
in mark_start_commit_inner() across already garbage-collected gco items.

The reclaimed gco links are explained with actually possible
out-of-order groups of events termination due to the Last_Error.
This patch reinforces the correct ordering to perform
finish_event_group's cleanup actions, incl unlinking gco:s
from the active list.

sql/rpl_parallel.h

@@ -90,6 +90,9 @@ struct group_commit_orderer {
     FORCE_SWITCH= 2
   };
   uint8 flags;
+#ifndef DBUG_OFF
+  bool gc_done;
+#endif
 };