Faster room joins: Device list cache not flushed when it is discovered that we incorrectly thought we shared a room with a remote user for some period in the past

This issue has been migrated from [#13887](https://github.com/matrix-org/synapse/issues/13887).

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See the skipped `TestPartialStateJoin/Device_list_tracking/Device_list_tracking_for_user_incorrectly_believed_to_be_in_room_when_they_rejoin_before_the_partial_state_join_completes` complement test.

That test case is roughly:
1. A room starts with `@charlie:remote`, `@derek:remote` (moderator) and `@fred:remote` (admin)
2. `@fred:remote` leaves the room.
3. `@alice:local` partial state joins the room.
4. `@elsie:remote` joins the room.
5. `@alice:local` does a `/keys/query` request and the local homeserver caches `@elsie:remote`'s device list.
6. `@fred:remote` "bans" `@derek:remote`, citing their join event as auth. The local homeserver does not know that `@fred:remote` is no longer in the room and accepts the event.
7. `@derek:remote` kicks `@elsie:remote`. The local homeserver incorrectly rejects the kick because it thinks `@derek:remote` has been banned. `@elsie:remote` stops notifying the local homeserver about device list updates.
8. `@elsie:remote` updates their device list.
9. `@elsie:remote` rejoins the room (or joins another shared room) and will notify the local homeserver about device list updates again.
10. The partial state join completes and `@elsie:remote` is discovered to not have been in the room / shared a room the whole time.
11. (bad) If `@alice:local` does a /keys/query request, a stale response is returned.

Start with the skipped TestPartialStateJoin/Device_list_tracking/Device_list_tracking_for_user_incorrectly_believed_to_be_in_room_when_they_rejoin_before_the_partial_state_join_completes complement test and trace the /keys/query request through the described device-list tracking flow. Done means the stale device-list response is no longer returned after the partial-state join discovers that the users did not share a room.

Written by the indexing model from the issue text.