Support room versioning

multiple rooms need to merged into one, with their timeline stitched together:
- [ ] in timelineFragment, the nextId and previousId fields should contain the full key including the room id rather than just the fragment id. This way their pointers can point across rooms.
- [ ] the roomSummary store gains a `previousRoomVersions` field, containing an array with the room ids
- [ ] when receiving a tombstone event:
- [ ] update the room summary: change the room id of the room summary to the new, superseding room id while adding the current room id to the previousRoomVersions field.
- [ ] create a new timeline fragment for the new room and point the old timeline fragment to it
- [ ] how do we handle updates to replaced rooms? by default it wouldn't find the room anymore and hence create a new one. We want to store the update except for the room summary, which should only be changed by the last room.