feat(event cache): reload a linked chunk from a store

[bnjbvr]

This implements the following:

• add a new trait method to reload a linked chunk from the store. In particular, we want to distinguish reloading when there's nothing in the store (could happen if we never saved a chunk before) from reloading a non-empty chunk. The reason is, that there's an invariant in the linked chunk code that when it's empty, a linked chunk has a single empty Events chunk; we let the caller maintain that invariant by creating the linked chunk in this case.
• implement that trait for the sqlite store; it's trivial, since the sqlite store had to implement similar functionality just so as to be tested.
• implement that trait for the in-memory store; while not complicated, it's a bit unwieldy because the in-memory store keeps one line per item in a chunk, instead of storing the entire items vector. But still, relatively easy to manage :-)
• adds integration tests at the trait level for the event cache store, for handling and reloading a chunk at the same time. Moves a bit of code from the sqlite testing framework, to help with creating and checking synthetic events.

Part of #3280

[Hywan]

Looks great to me!

[Hywan]

Good job hywan.

[codecov]

Codecov Report

Attention: Patch coverage is 89.07563% with 13 lines in your changes missing coverage. Please review.

Project coverage is 85.13%. Comparing base (685386d) to head (771cb00).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
...s/matrix-sdk-common/src/linked_chunk/relational.rs	76.19%	10 Missing ⚠️
...rix-sdk-base/src/event_cache/store/memory_store.rs	77.77%	2 Missing ⚠️
crates/matrix-sdk-sqlite/src/event_cache_store.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4362      +/-   ##
==========================================
+ Coverage   85.10%   85.13%   +0.02%     
==========================================
  Files         280      280              
  Lines       30535    30654     +119     
==========================================
+ Hits        25988    26098     +110     
- Misses       4547     4556       +9     

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[bnjbvr]

@Hywan FWIW, I've added a small fix in the last commit (with a regression test). If we added an empty items chunk (which is totally fine, by contract), it would incorrectly lead to an invalid data error. Instead, when we find a chunk with no rows, we have to assume it's an empty events chunk. That's the change happening in the last commit.

[Hywan]

All good to me, thanks for the bug fix!