feat(event cache): enable persistent storage by default 😎

[bnjbvr]

This implements the following parts of #3280 (comment):

• implement a sqlite backend for the event cache store
• save events into the store as they come from sync
• reload data from the store and use it to prefill the linked chunks for all rooms

For getting a nice demo in the multiverse client, a few shortcuts have been taken in the last commit, but these should be relatively easy to solve.

The demo shows the following:

• open the client for the first time
• observe the focused room is mostly empty
• I manually back-paginate, getting some events back from the server
• then, close the client
• reopen it
• observe the events are still there (they were persisted on disk).

Screencast.From.2024-11-21.18-09-05.mp4

More to come next week.

[bnjbvr]

For what it's worth, to observers of this PR: this is being split, redesigned and reviewed in several smaller PRs, which all link to this one. This PR might be repurposed to represent the final step of the work done here, at some point.

[Hywan]

Is it useful to keep this PR open?

[bnjbvr]

Is it useful to keep this PR open?

I think I can repurpose it to become the "enable storage by default", by removing a few commits in there :-)

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 86.14%. Comparing base (4742aa2) to head (b7f6b19).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/event_cache/pagination.rs	89.47%	4 Missing ⚠️
crates/matrix-sdk/src/event_cache/room/mod.rs	95.23%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #4308    +/-   ##
========================================
  Coverage   86.14%   86.14%            
========================================
  Files         291      291            
  Lines       34308    34165   -143     
========================================
- Hits        29553    29431   -122     
+ Misses       4755     4734    -21     

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[bnjbvr]

This is now ready for review, as it passes all the tests on my machine (and hopefully in CI too!).

But we're likely not going to merge this any time soon, as it removes the feature toggle for the event cache's persisted storage. Instead, we'll likely keep the feature toggle for a bit (exact period TBD: one release? some weeks? something that we'll discuss internally), to avoid having to revert a large PR in case things go awfully bad.