Previously, unloaded events will show as "unknown event" and since the introduction of DbStore, there are more and more unloaded events as replied-to events in the timeline. This commit automatically loads the event from DbStore when it encounters an unknown event. Further work can be done to allow the user to fall back to client-server api after libkazv implements fetching one single event by id.

Type: add

Reply to a very old event in a room. Switch to another room and wait until the event is purged. Switch back to the original room, and check you can see the old event immediately as the replied-to event.