Relation event insertion breaks during backwards pagination if parent is on another page (if `supportsThreads: true`)

Nobody has claimed this yet.

Assume that we have a `PARENT` event a `CHILD` event which is an `m.replace` of `PARENT`, sent a long time after `PARENT` was sent (so there are some pages between the two events).

If I have `supportsThreads: true`, and I'm backfilling the room via `paginateEventTimeline` (going in backwards direction), I hit `CHILD` first, going through `addEventsToTimeline` -> `eventShouldLiveIn`, leading me to...

https://github.com/matrix-org/matrix-js-sdk/blob/b515cdbdbbe21d481409c677af365a5315fb6390/src/models/room.ts#L2171

Since I didn't hit `PARENT` yet (and it is not in the page that is currently being processed, i.e. `events`), `parentEvent` is not found, causing the following case to fail (I think this case is the one that *should* be hit, although we don't have parent so we can't recur):

https://github.com/matrix-org/matrix-js-sdk/blob/b515cdbdbbe21d481409c677af365a5315fb6390/src/models/room.ts#L2174-L2177

All other branches fail, causing the function to return:

https://github.com/matrix-org/matrix-js-sdk/blob/b515cdbdbbe21d481409c677af365a5315fb6390/src/models/room.ts#L2198-L2204

This omits `CHILD` from the timeline (and I think shows `PARENT` instead?).

(I'm probably going to just disable `supportsThreads` to workaround this for myself.)

Start in src/models/room.ts at paginateEventTimeline, addEventsToTimeline, and eventShouldLiveIn, especially the relation handling around the linked lines. Reproduce backwards pagination with supportsThreads: true when a child m.replace event appears before its parent on a later page. Done means the child is retained in the timeline instead of being omitted when its parent is not in the current page.

Written by the indexing model from the issue text.