This is the last part of the event storage implementation in kazv. It purges previous events via a timer, and it backfills when the timeline is scrolled to the beginning. With this, we will hopefully solve the problem of long session loading time and high memory usage.

Type: add

CCBUG: https://iron.lily-is.land/T165

Set environment variable export QT_LOGGING_RULES='moe.kazv.mxc*=true'.
Load a session with kazv. Open a room.
Wait for enough time (5mins) and verify that the log contains Purge room events stat:.
Verify that the opened room is not purged.
Open another room. Verify that the events in that room is purged.
Verify that more events will be loaded when you scroll to the top.
Close and load the session again. Verify it takes much less time to load.
Verify that number of events saved to the storage in the log is much fewer.