Olm sessions broken by use of outdated cache

[richvdh]

We recently found and fixed a bug in Element X iOS (matrix-org/matrix-rust-sdk#3110) which caused Olm sessions to be corrupted due to lack of cache invalidation when control switched between the main process and the NSE process.

According to @BillCarsonFr, Element iOS suffers the same problem; we should fix this as it causes users on other platforms to see UTD errors.

[richvdh]

Estimated as "S" assuming we just delete the crypto store cache when we come to the foreground

[BillCarsonFr]

• FFI Bindings: Expose CryptoStore clear caches matrix-org/matrix-rust-sdk#3462
• Invalidate crypto store cache when entering foreground matrix-org/matrix-ios-sdk#1854

[BillCarsonFr]

In RC #7798

[richvdh]

According to @BillCarsonFr, this is still a potential problem.

The current workaround on classic ios is that we "refresh" the olm machine when the app enters foreground
So the workaround limits the possibilities of outdated cache but not 100% of them

[richvdh]

Related: #7751