[Meta] `SlidingSyncRoom` tech debt

[Hywan]

We have a non-neligeable tech debt about matrix_sdk::sliding_sync::SlidingSyncRoom.

It holds data that are not shared with the matrix_sdk::Room. Ideally, we want SlidingSyncRoom to not exist.

The idea is to clean this as much as possible.

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Blocked by:

• We need the event cache ([meta] Event Cache API 🕸 #3058) to have a storage, so that we can get rid off SlidingSyncRoom::timeline_queue,
• feat(sdk): Event cache experimental store: LinkedChunk #3166
• feat(ui): Propagate RoomEventCache's VectorDiff to Timeline #3512