Fix Hot Standby feedback sending when streaming busily.

Commit 6f60fdd7 accidentally removed a
call to XLogWalRcvSendHSFeedback() after flushing received WAL to disk.
The consequence is that when walsender is busy streaming WAL, it doesn't
send HS feedback messages. One is sent if nothing is received from the
master for 100ms, but if there's a steady stream of WAL, it never happens.

Backpatch to 9.3.

Andres Freund and Amit Kapila

src/backend/replication/walreceiver.c

@@ -1007,7 +1007,10 @@ XLogWalRcvFlush(bool dying)
 
 		/* Also let the master know that we made some progress */
 		if (!dying)
+		{
 			XLogWalRcvSendReply(false, false);
+			XLogWalRcvSendHSFeedback(false);
+		}
 	}
 }