Event cache: handle latest preview event

[bnjbvr]

Now that we do have basic storage for the event cache, we could store the latest computed "preview" event (to be show in the room list, among other things).

Some rough list of implementation steps:

• Store the latest preview event in the event cache store, and restore it on demand
  • Fuse the "room info" subscriber with a subscriber on the latest preview event from the event cache
  • Make sure to also include local echoes as possible latest preview events. Product decisions have to be made, whether to advantage wedged local echoes over more-recent messages, for instance.
• Event cache: Move decryption retry in there #3872, to decrypt the latest preview event.
• Better handle edits, when they apply to the latest event (e.g. this issue)
• Redaction of latest timeline event is not reflected in the latest preview room event #2266

Part of #3058.