refactor(crypto): Make memory store behave more like other stores

[BillCarsonFr]

Attempt to make the memory store behave more like other stores.
This is following reports of Playwright flaky tests that are showing some panics of the memory store (used by the bots not the tested app).

The idea is to make the memory store works more like other stores by removing unwrap()s, serializing/deserialising structures instead of keep them around to be mutated by everyone. Add a global save_changes lock

PRO

Will behave more like other stores (might help and make the tests less flaky)

CONS

Might be slower

Breakdown

Best reviewed commit per commit (will squash at the end):

• Replace unwrap with expect 17fa2c7

• Serialize account and cache static account data b7ac36f

• Serialize/Deserialise olm sessions 7c87d13

• Ser/Deser inbound group sessions 9262c30

• Add a global save change lock similar to other stores 70c2baf

• Public API changes documented in changelogs (optional)

Signed-off-by:

[andybalaam]

Why squash at the end?

[codecov]

Codecov Report

Attention: Patch coverage is 98.61111% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.42%. Comparing base (4c4dd03) to head (27ebbff).

Files with missing lines	Patch %	Lines
crates/matrix-sdk-crypto/src/store/memorystore.rs	98.61%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4558   +/-   ##
=======================================
  Coverage   85.42%   85.42%           
=======================================
  Files         286      286           
  Lines       32213    32247   +34     
=======================================
+ Hits        27517    27548   +31     
- Misses       4696     4699    +3     

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[andybalaam]

This looks sensible to me.