feat(base) Implement RelationalLinkedChunk

[Hywan]

A book floating in space in front of a black hole.

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A RelationalLinkedChunk is like a LinkedChunk but with a relational
layout, similar to what we would have in a database.

This is used by memory stores. The idea is to have a data layout that
is similar for memory stores and for relational database stores, to
represent a LinkedChunk.

This type is also designed to receive Update. Applying Updates
directly on a LinkedChunk is not ideal and particularly not trivial
as the Updates do not match the internal data layout of the
LinkedChunk, they have been designed for storages, like a relational
database for example.

This type is not as performant as LinkedChunk (in terms of memory
layout, CPU caches etc.). It is only designed to be used in memory
stores, which are mostly used for test purposes or light usages of the
SDK.

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• Must be merged after feat(base): Add EventCacheStore::handle_linked_chunk_updates #4299 as it contains its commits
• Address [meta] EventCache storage #3280

[codecov]

Codecov Report

Attention: Patch coverage is 85.08772% with 17 lines in your changes missing coverage. Please review.

Project coverage is 85.11%. Comparing base (e5ca44b) to head (c788221).
Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
...s/matrix-sdk-common/src/linked_chunk/relational.rs	87.35%	11 Missing ⚠️
...rix-sdk-base/src/event_cache/store/memory_store.rs	83.33%	3 Missing ⚠️
crates/matrix-sdk-common/src/linked_chunk/mod.rs	66.66%	2 Missing ⚠️
...es/matrix-sdk-base/src/event_cache/store/traits.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4298      +/-   ##
==========================================
+ Coverage   85.09%   85.11%   +0.01%     
==========================================
  Files         274      275       +1     
  Lines       30183    30280      +97     
==========================================
+ Hits        25684    25772      +88     
- Misses       4499     4508       +9     

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[bnjbvr]

This code is likely wrong, given that the store method should include a reference to a RoomId (sorry, should've caught that during review of the patch adding the trait method, but noticed the problem only while doing the implementation for the SQLite backend…).

[bnjbvr]

lgtm! minor comments so approving to avoid back-and-forth, and I've included some ideas for bigger improvements (a single inner lock for the memory store; tweak the signature of handle_linked_chunk_updates) that can be done here or as follow-up PRs. Thanks!