Fix state_after for left rooms including post-leave lazy-loaded memberships

[barodeur]

The bug

TL;DR: with the experimental MSC4222 implementation enabled (use_state_after) and lazy-loading of room members, an incremental /sync could disclose state from after the user's leave in a left room's state_after.

1. Alice syncs with lazy_load_members: true and use_state_after=true.
2. Bob sends a message in a room they share.
3. Alice leaves the room.
4. Bob updates his per-room displayname
5. Alice does an incremental sync covering steps 2–3. Alice's state_after contains Bob's post-leave membership event from step 4

Alice should not see the new per-room display name of Bob.

The fix

Copy what has been done for _compute_state_delta_for_full_sync: pass joined down and, for rooms the user is no longer joined to, fetch the memberships as of end_token via state groups (get_state_ids_at) instead of current state.

Pull Request Checklist

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