feat(common): Introduce Chunk::lazy_previous in the LinkedChunk

[Hywan]

This patch introduces Chunk::lazy_previous which is a key feature to support lazy-loading of a LinkedChunk. When a chunk is loaded, if it is the first, it keeps in memory whether it has a previous chunk or not. Thus, it is possible to insert new chunk in front of the LinkedChunk, and Updates will correctly continue to link chunks between them (with NewItemsChunk and NewGapChunk).

Example, imagine the following chunks: [0] <-> [1] <-> [2]. If [2] is the only one being loaded. Then its previous chunk, [1], is loaded from the store (because [2]'s previous is [1] in the store). Then [1] is replaced by [3] and [4]. We get this: [4] <-> [3] <-> [1] <-> [2]. If the Update::New*Chunk for [4] doesn't contain a previous, the store is out of sync: in the store, [4] has no previous, but [0] still has [1] for its next.

With this lazy_previous, the links are correctly computed.

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• Address [meta] EventCache storage #3280
• Extracted from feat(sdk): Implement EventCache lazy-loading #4632

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.84%. Comparing base (1c11497) to head (00637f3).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-common/src/linked_chunk/mod.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4675      +/-   ##
==========================================
- Coverage   85.84%   85.84%   -0.01%     
==========================================
  Files         292      292              
  Lines       33634    33638       +4     
==========================================
+ Hits        28873    28876       +3     
- Misses       4761     4762       +1     

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[bnjbvr]

Seeing how it's used in #4632, makes sense!