Gracefully disconnect MQTT (#23)

* Gracefully disconnect MQTT

* Add MG_F_SEND_AND_CLOSE flag to disconnect

Author: guywithaview

src/mgos_mqtt_conn.c

@@ -490,7 +490,8 @@ void mgos_mqtt_conn_disconnect(struct mgos_mqtt_conn *c) {
   if (c == NULL) return;
   c->reconnect_timeout_ms = -1;  // Prevent reconnect.
   if (c->nc != NULL) {
-    c->nc->flags |= MG_F_CLOSE_IMMEDIATELY;
+    mg_mqtt_disconnect(c->nc);
+    c->nc->flags |= MG_F_SEND_AND_CLOSE;
   }
 }