Reset walsender ps title in the main loop

When in streaming mode we can never get out, so it will never
be required, but after a base backup (or other operations)
we can get back to the loop, so the title needs to be cleared.

src/backend/replication/walsender.c

@@ -179,6 +179,8 @@ WalSndHandshake(void)
 	{
 		int			firstchar;
 
+		set_ps_display("idle", false);
+
 		/* Wait for a command to arrive */
 		firstchar = pq_getbyte();