fix(really-core): Remove 'Thread.sleep' from PersistentModelStoreSpec

- Modify `PersistentModelStore` to take `persistenceId` as a parameter
- Modify `PersistentModelStore` to handle `GetState` message, that for testing purpose only
you don't need extend `BeforeAndAfterAll` because `BaseActorSpec` already extend  `BeforeAndAfterAll`