Stop table sync workers when subscription relation entry is removed

When a table sync worker is in waiting state and the subscription table
entry is removed because of a concurrent subscription refresh, the
worker could be left orphaned.  To avoid that, explicitly stop the
worker when the pg_subscription_rel entry is removed.

Reported-by: Masahiko Sawada <sawada.mshk@gmail.com>

src/backend/commands/subscriptioncmds.c

@@ -600,6 +600,8 @@ AlterSubscription_refresh(Subscription *sub, bool copy_data)
 
 			RemoveSubscriptionRel(sub->oid, relid);
 
+			logicalrep_worker_stop(sub->oid, relid);
+
 			namespace = get_namespace_name(get_rel_namespace(relid));
 			ereport(NOTICE,
 					(errmsg("removed subscription for table %s.%s",