event cache: internalize handling of the account data

[bnjbvr]

This is a relatively simple change: this moves handling the fully read marker for a given room into the event cache, so listeners can react to a specific event for that, instead of handling themselves all the account data changes.

Not sure if that's a better API than just returning all the account data changes, so I would love feedback on this. At least it's sufficient for the UI timeline use case, and so it seems sufficient right now (and we could make it different later).

Part of #3058.

[codecov]

Codecov Report

Attention: Patch coverage is 84.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 83.60%. Comparing base (ac0bc95) to head (9885dfe).
Report is 2 commits behind head on main.

Files	Patch %	Lines
crates/matrix-sdk/src/event_cache/mod.rs	69.23%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3276      +/-   ##
==========================================
- Coverage   83.61%   83.60%   -0.01%     
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  Files         238      238              
  Lines       24585    24571      -14     
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- Hits        20556    20542      -14     
  Misses       4029     4029              

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[andybalaam]

Looks good, and I prefer a more-specific API in general terms. I don't think I understand well enough to comment on this exact choice of API.

[Hywan]

Thanks for the work. However, I'm not sure about this changes. The idea is to remove RoomEventCacheUpdate to replace it by a single Stream<Item = Vec<VectorDiff<SyncTimelineEvent>>>, and I don't know how this feature could fit in the next API.

[bnjbvr]

The timeline will always need to listen to those account data changes, or at least the "read marker has been updated" event, so:

• either we say the event cache only strictly streams event updates, and does not pass forward the "read marker has been updated" event — which means that the timeline now needs to listen to 1. the event cache updates for a room, 2. also the JoinedRoomUpdates for a room, which seems messy
• or the event cache has two kinds of updates: strict event updates (via Vec<VectorDiff>), or other kind of updates (like the one introduced in this PR)
• or instead of streaming SyncTimelineEvent, we stream a thin wrapper that also contains a bool indicating whether this event is the target of the read marker. It sounds a bit wasteful to me, since it'll be false for most updates, and it's not clear that this will cleanly extend to other info streamed from the event cache, e.g. about read receipts

[Hywan]

After a discussion about the possible evolution of this API in private, I'm approving this PR :-). Thanks!