Save and restore event cache to/from persistent storage

[kevinaboos]

Now that a user's login session is properly persisted to/from storage on all platforms, the next step is to:

1. Serialize the actual set of all timeline events (as a list of SyncTimelineEvents) upon app exit, or perhaps upon another lifecycle stage.
2. Deserialize them and repopulate the event cache before synchronizing any timelines, using the EventCache::add_initial_events()
   • We just removed an erroneous invocation of this function in fde5d29

[kevinaboos]

This is blocked on the Matrix Rust SDK, which currently doesn't fully implement EventCache features that allow the client to customize how events are extracted from and imported into the cache.