feat(event cache): have a debug representation of a linked chunk and use it in multiverse

[bnjbvr]

I think that this debug representation may show useful to understand issues. Just from using it a few minutes, I've identified a few issues related to storage:

• we shouldn't have empty items chunks; they should be removed, or even better, reused.
• we may have multiple prev-batch token that represent the same events, e.g. if we sync'd a room with timeline_limit=1, then with timeline_limit=20. Not sure if there's anything obvious to do about that.
  • edit: yes there is! do not store a prev-batch token when all the events (at least 1) returned from the backpagination are known. Should be a sufficiently good heuristic 🧠

[codecov]

Codecov Report

Attention: Patch coverage is 55.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 85.29%. Comparing base (05d46e6) to head (43632c5).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/event_cache/room/events.rs	0.00%	7 Missing ⚠️
crates/matrix-sdk/src/event_cache/room/mod.rs	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4411      +/-   ##
==========================================
- Coverage   85.31%   85.29%   -0.03%     
==========================================
  Files         283      283              
  Lines       31424    31435      +11     
==========================================
+ Hits        26810    26812       +2     
- Misses       4614     4623       +9     

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[stefanceriu]

Super nice! Any chance we can expose this through FFI? We could dump the contents into a separate screen like we do for message sources.

[bnjbvr]

@stefanceriu Done in the last commit; this is a simple change, so not re-requesting a review just for this :) Thanks for the review!