mark loop as shuttingdown earlier in shutdown_cb

`shutdown_trigger_close_cb` is not called in the main loop since
queued events in the `loop->async_trigger`, including loop teardown
(shutdown_server) are processed first, before the `uv_close` callback
is executed..

In order to pass the information to the queued events, it is necessary
to set the flag earlier in the process and not wait for the `uv_close`
callback to trigger.

lib/isc/loop.c

@@ -140,8 +140,6 @@ static void
 shutdown_trigger_close_cb(uv_handle_t *handle) {
 	isc_loop_t *loop = uv_handle_get_data(handle);
 
-	loop->shuttingdown = true;
-
 	isc_loop_detach(&loop);
 }
 
@@ -167,6 +165,9 @@ shutdown_cb(uv_async_t *handle) {
 	/* Make sure, we can't be called again */
 	uv_close(&loop->shutdown_trigger, shutdown_trigger_close_cb);
 
+	/* Mark this loop as shutting down */
+	loop->shuttingdown = true;
+
 	if (DEFAULT_LOOP(loopmgr) == CURRENT_LOOP(loopmgr)) {
 		/* Stop the signal handlers */
 		isc_signal_stop(loopmgr->sigterm);