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
can u check this http://www.scalatest.org/user_guide/sharing_fixtures#withFixtureOneArgTest  instead to `var`