Avoid excessive Hot Standby feedback messages.

Without this patch, when wal_receiver_status_interval=0, indicating that no
status messages should be sent, Hot Standby feedback messages are instead sent
extremely frequently.

Fujii Masao, with documentation changes by me.

doc/src/sgml/config.sgml

@@ -2137,9 +2137,10 @@ SET ENABLE_SEQSCAN TO OFF;
        <para>
         Specifies whether or not a hot standby will send feedback to the primary
         about queries currently executing on the standby. This parameter can
-        be used to eliminate query cancels caused by cleanup records, though
-        it can cause database bloat on the primary for some workloads.
-        The default value is <literal>off</literal>.
+        be used to eliminate query cancels caused by cleanup records, but 
+        can cause database bloat on the primary for some workloads.
+        The default value is <literal>off</literal>.  Feedback messages will not
+        be sent more frequently than once per <varname>wal_receiver_status_interval</>.
        </para>
       </listitem>
      </varlistentry>

src/backend/replication/walreceiver.c

@@ -651,7 +651,7 @@ XLogWalRcvSendHSFeedback(void)
 	 * If the user doesn't want status to be reported to the master, be sure
 	 * to exit before doing anything at all.
 	 */
-	if (!hot_standby_feedback)
+	if (wal_receiver_status_interval <= 0 || !hot_standby_feedback)
 		return;
 
 	/* Get current timestamp. */