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
Merge pull request #1 from ashihaby/fixes

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