Fix race condition when changing synchronous_standby_names

A momentary window exists when synchronous_standby_names
changes that allows commands issued after the change to
continue to act as async until the change becomes visible.
Remove the race by using more appropriate test in syncrep.c

Author: Asim Rama Praveen and Ashwin Agrawal
Reported-by: Xin Zhang, Ashwin Agrawal, and Asim Rama Praveen
Reviewed-by: Michael Paquier, Masahiko Sawada

src/backend/replication/syncrep.c

@@ -156,11 +156,9 @@ SyncRepWaitForLSN(XLogRecPtr lsn, bool commit)
 		mode = Min(SyncRepWaitMode, SYNC_REP_WAIT_FLUSH);
 
 	/*
-	 * Fast exit if user has not requested sync replication, or there are no
-	 * sync replication standby names defined. Note that those standbys don't
-	 * need to be connected.
+	 * Fast exit if user has not requested sync replication.
 	 */
-	if (!SyncRepRequested() || !SyncStandbysDefined())
+	if (!SyncRepRequested())
 		return;
 
 	Assert(SHMQueueIsDetached(&(MyProc->syncRepLinks)));