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/PostgresStorageAdapter.spec.js

@@ -12,7 +12,7 @@ const dropTable = (client, className) => {
 describe_only_db('postgres')('PostgresStorageAdapter', () => {
   const adapter = new PostgresStorageAdapter({ uri: databaseURI })
   beforeEach(() => {
-    return adapter.dropDatabase();
+    return adapter.deleteAllClasses();
   });
 
   it('schemaUpgrade, upgrade the database schema when schema changes', done => {