Fix slowdown in TAP tests due to recent walreceiver change.

Commit 05a7be93 changed the timing of the first reply sent by a
walreceiver, which caused a few TAP tests that call wait_for_catchup()
when they haven't actually streamed anything yet to wait ~10 seconds
(wal_receiver_status_interval).

Before commit 05a7be93 the initial reply was sent after 100ms, but
there's no reason not to send it immediately as a slight improvement.
Do the same for HS feedback for consistency.

Author: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/742545.1668377284%40sss.pgh.pa.us

src/backend/replication/walreceiver.c

@@ -433,6 +433,10 @@ WalReceiverMain(void)
 			for (int i = 0; i < NUM_WALRCV_WAKEUPS; ++i)
 				WalRcvComputeNextWakeup(i, now);
 
+			/* Send initial reply/feedback messages. */
+			XLogWalRcvSendReply(true, false);
+			XLogWalRcvSendHSFeedback(true);
+
 			/* Loop until end-of-streaming or error */
 			for (;;)
 			{