repmgrd: prevent standby connection handle from going stale

If monitoring history not in use, there's no activity on the standby's
connection handle, so if e.g. the standby is restarted, PQstatus()
never returns CONNECTION_BAD and repmgrd never notices the connection
is stale. Therefore execute a throw-away statement at "monitor_interval_secs".

dbutils.c

@@ -3847,6 +3847,21 @@ is_server_available_params(t_conninfo_param_list *param_list)
 }
 
 
+
+/*
+ * Simple throw-away query to stop a connection handle going stale
+ */
+void
+connection_ping(PGconn *conn)
+{
+	PGresult   *res = PQexec(conn, "SELECT TRUE");
+
+	PQclear(res);
+	return;
+}
+
+
+
 /* ==================== */
 /* monitoring functions */
 /* ==================== */