Consistently use subscription name as application name

The logical replication apply worker uses the subscription name as
application name, except for table sync.  This was incorrectly set to
use the replication slot name, which might be different, in one case.
Also add a comment why the other case is different.

Author: petere

src/backend/replication/logical/tablesync.c

@@ -817,6 +817,11 @@ LogicalRepSyncTableStart(XLogRecPtr *origin_startpos)
 						MySubscription->oid,
 						MyLogicalRepWorker->relid);
 
+	/*
+	 * Here we use the slot name instead of the subscription name as the
+	 * application_name, so that it is different from the main apply worker,
+	 * so that synchronous replication can distinguish them.
+	 */
 	wrconn = walrcv_connect(MySubscription->conninfo, true, slotname, &err);
 	if (wrconn == NULL)
 		ereport(ERROR,

src/backend/replication/logical/worker.c

@@ -1595,7 +1595,7 @@ ApplyWorkerMain(Datum main_arg)
 		origin_startpos = replorigin_session_get_progress(false);
 		CommitTransactionCommand();
 
-		wrconn = walrcv_connect(MySubscription->conninfo, true, myslotname,
+		wrconn = walrcv_connect(MySubscription->conninfo, true, MySubscription->name,
 								&err);
 		if (wrconn == NULL)
 			ereport(ERROR,