Make Cache constructor exception free

fixes #74

* Initial "join room" feature.
* React correctly to remotely joined rooms.
* Leaving rooms implemented both locally using the room menu
   in nheko, and reacting properly when leaving a room remotely 
   from another client.

- Adds detection for duplicate events

- Rooms without any history will be shown.
- Room's state will be kept in sync and any updates will be visible.