[timeline/event-cache] a timeline clear should cause a cached events read from the event cache

[bnjbvr]

Rough notes from an internal chat room:

FYI, there are multiple reports of the timeline "lagging behind" the room event cache updates, like https://github.com/element-hq/element-x-ios-rageshakes/issues/1674#issuecomment-2045118673

This happens because the timeline can't keep up with the rate of room event cache updates, which is super surprising? the timeline shouldn't run code for very long, in theory, so there's more to untangle there. We'd need to investigate a bit more to understand why this happens, and if there are ways to mitigate that.

One obvious way would be to add an additional task that just forwards cache updates into an unbounded queue, but I wouldn't add more concurrency to the mix just yet.

Now, I think that was confuses people is they see old events, and interpret this as missing chunks. My WIP theory is the following:

• back-pagination is happening in the background
• the timeline is cleared because of a lag behind the room event cache updates
• back-pagination finishes, and old events are appended to the timeline because of that.

Now, this is the same failure mode as in the "when ignoring a user, you may see old items", with the same reason, except #2 is a timeline clear because we ignored a user.

In all cases, we expect the caller to retrigger back-pagination manually, but in this case it would be wrong, because there are already back-paginated events in the timeline.

oh this ain't gonna work, since it's event cache handling the back-pagination state now 🤦

When clearing the timeline, we should append all the events from the event cache

Part of #3058

[bnjbvr]

After thinking about it, we only need to do this upon lag behind the event cache. In other cases, we might be clearing the room with a different intent, so retrieving the events from the cache would not be the right move.