Follow Node.js callback pattern

lib/api/middlewares/user.js

@@ -8,11 +8,12 @@ module.exports = function user (metadataBackend) {
     return function userMiddleware (req, res, next) {
         res.locals.user = getUserNameFromRequest(req, cdbRequest);
 
-        getUserId(metadataBackend, res.locals.user, function (userId) {
-            if (userId) {
-                res.locals.userId = userId;
+        metadataBackend.getUserId(res.locals.user, (err, userId) => {
+            if (err || !userId) {
+                return next();
             }
-            return next();
+
+            res.locals.userId = userId;
         });
     };
 };
@@ -20,12 +21,3 @@ module.exports = function user (metadataBackend) {
 function getUserNameFromRequest (req, cdbRequest) {
     return cdbRequest.userByReq(req);
 }
-
-function getUserId (metadataBackend, userName, callback) {
-    metadataBackend.getUserId(userName, function (err, userId) {
-        if (err) {
-            return callback();
-        }
-        return callback(userId);
-    });
-}