`JoinedRoom` will now contain both a mandatory live timeline reference and all the functionality associated to it.
`BaseRoom` on the other hand will contain only functionality that's shared for both joined and not joined rooms.
`NotJoinedRoom` is a wrapper around `RoomPreviewInfo` data and a possible local `BaseRoom`, if it exists.
The `RustRoomFactory` cache is now gone since the persistent event cache should have the same effect.
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