Adds index on _Role name property (#3586)

* Adds index on _Role name property

In order to avoid having different _Role objects with the same name, adding an index on the name property of _Role is necessary.

Fixes #3579

* Uses throw instead of Promise.reject when enforcing unique indexes

* Fixes wrong sorting of results in schemas tests

spec/Schema.spec.js

@@ -494,7 +494,8 @@ describe('SchemaController', () => {
 
   it('creates non-custom classes which include relation field', done => {
     config.database.loadSchema()
-    .then(schema => schema.addClassIfNotExists('_Role', {}))
+    //as `_Role` is always created by default, we only get it here
+    .then(schema => schema.getOneSchema('_Role'))
     .then(actualSchema => {
       const expectedSchema = {
         className: '_Role',