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
I think it is better to add new Actor extend this actor for testing and add this message on it. Like `CollectionViewMaterializer`