Event cache: Move decryption retry in there

[bnjbvr]

Right now, it's the Timeline API that takes care of retrying decryption of an event. This has a few bad consequences:

• retrying decryption, while it could be done in other contexts than the Timeline API, can only happen there
• some observers (e.g. the UTD hook) want to know when a previously-UTD event could be decrypted; this happens in the Timeline code too, which is an abstraction leak
• some other code basically retries decryption on its own: the latest event code in the base crate does that

I'm proposing that we move all of this over to the Event Cache. An event in a linked chunk / on disk would be replaced by its decrypted equivalent, if needs be, and we can implement a few enhancements mentioned in a few other issues too.

• After re-decrypting an event correctly, emit a new kind of room event cache update for the decrypted event (and don't forget to update the other in-memory cache)
• Historical messages decrypt on cache reloading, but not before #3768 (comment)
• Once we have some basic storage (doesn't need reconciliation etc.) in [meta] EventCache storage #3280, put the latest event logic in there, and remove it from the base crate
• timeline/event-cache: try to re-decrypt bundled events #4106

Part of #3058.