feat(event cache): write updates from the event cache to storage

[bnjbvr]

This is one direction¹ of interacting between the event cache and the storage backend. I think this is fine design… as long as we don't have any other process writing into the cache, in which case we'll need to reset at different boundaries.

The main idea in this PR is to use RoomEventCacheState::with_events_mut(func: impl FnMut(&mut RoomEvents)) to perform any write to the underlying RoomEvent data structure. The function takes care of maintaining the storage, after updates have been performed.

Additionally: this behavior isn't enabled by default; callers have to call EventCache::enable_storage() when they start using the event cache.

I've put the RoomEventCacheState data structure in a private module, so that even the same module can't access its private fields, and we don't make mistakes by accidentally misusing events.

Part of #3280. Split and reworked from #4308.

Footnotes

1. not the band ↩

[codecov]

Codecov Report

Attention: Patch coverage is 90.66667% with 7 lines in your changes missing coverage. Please review.

Project coverage is 85.13%. Comparing base (e1f0037) to head (884bb5c).

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/event_cache/pagination.rs	82.60%	4 Missing ⚠️
crates/matrix-sdk/src/event_cache/mod.rs	76.92%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4347      +/-   ##
==========================================
+ Coverage   85.10%   85.13%   +0.02%     
==========================================
  Files         280      280              
  Lines       30562    30595      +33     
==========================================
+ Hits        26010    26046      +36     
+ Misses       4552     4549       -3     

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