refactor: server code (#2545)

+ Remove duplicated await
+ Replace some legacy codes with ES6
+ Fix some of eslint problems

server/setup.js

@@ -378,7 +378,7 @@ module.exports = () => {
   // ----------------------------------------
 
   app.use(function (req, res, next) {
-    var err = new Error('Not Found')
+    const err = new Error('Not Found')
     err.status = 404
     next(err)
   })