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
feat(really-core): Implements Receptionist Actor
- Creates RequestDelegate (per request) for every request sent from
  WebsocketHandler
- handles Accounting (no. of requests and the frequency of requests)
- Adds stubs for RequestDelegate
- Adds Test Utililty Actor