RIO-31 feat(really-core): Implement Reference Field Type 

- refactor CollectionActor to be FSM
- handle reference field on create request on CollectionActor
- fix test cases fro CollectionActor
- add reference filed
- implement CollectionActorTest to extend CollectionActor and handle test messages like `GetState`
- fix test cases for CollectionViewMaterializer, ModelLoader and GorillaEventCenter
I think these fields `_r` and `_rev` should be constants in the model, so we can easily rename it later if needed