Even faster tests (#4868)

* Various improvements in test name / de-duplications

* Reverts to class by class deletion, introduced fast mode that just delete data for mongo

- Speeds up are incredible Executed 1695 of 1713 specs INCOMPLETE (18 PENDING) in 4 mins 19 secs.

* Adds documentation about the deleteEverything

spec/MongoStorageAdapter.spec.js

@@ -9,7 +9,7 @@ const databaseURI = 'mongodb://localhost:27017/parseServerMongoAdapterTestDataba
 describe_only_db('mongo')('MongoStorageAdapter', () => {
   beforeEach(done => {
     new MongoStorageAdapter({ uri: databaseURI })
-      .dropDatabase()
+      .deleteAllClasses()
       .then(done, fail);
   });