Properly set base backup backends to active in pg_stat_activity

When walsenders were included in pg_stat_activity, only the ones
actually streaming WAL were listed as active when they were active. In
particular, the connections sending base backups were listed as being
idle. Which means that a regular pg_basebackup would show up with one
active and one idle connection, when both were active.

This patch updates to set all walsenders to active when they are
(including those doing very fast things like IDENTIFY_SYSTEM), and then
back to idle. Details about exactly what they are doing is available in
pg_stat_replication.

Patch by me, review by Michael Paquier and David Steele.

src/backend/replication/walsender.c

@@ -1504,6 +1504,9 @@ exec_replication_command(const char *cmd_string)
 	initStringInfo(&reply_message);
 	initStringInfo(&tmpbuf);
 
+	/* Report to pgstat that this process is running */
+	pgstat_report_activity(STATE_RUNNING, NULL);
+
 	switch (cmd_node->type)
 	{
 		case T_IdentifySystemCmd:
@@ -1555,6 +1558,9 @@ exec_replication_command(const char *cmd_string)
 				ereport(ERROR,
 						(errmsg("cannot execute SQL commands in WAL sender for physical replication")));
 
+			/* Report to pgstat that this process is now idle */
+			pgstat_report_activity(STATE_IDLE, NULL);
+
 			/* Tell the caller that this wasn't a WalSender command. */
 			return false;
 
@@ -1570,6 +1576,9 @@ exec_replication_command(const char *cmd_string)
 	/* Send CommandComplete message */
 	EndCommand("SELECT", DestRemote);
 
+	/* Report to pgstat that this process is now idle */
+	pgstat_report_activity(STATE_IDLE, NULL);
+
 	return true;
 }
 
@@ -2089,9 +2098,6 @@ WalSndLoop(WalSndSendDataCallback send_data)
 	last_reply_timestamp = GetCurrentTimestamp();
 	waiting_for_ping_response = false;
 
-	/* Report to pgstat that this process is running */
-	pgstat_report_activity(STATE_RUNNING, NULL);
-
 	/*
 	 * Loop until we reach the end of this timeline or the client requests to
 	 * stop streaming.