Before this change, idle compaction is triggered every 1 second
if the incoming message stream is idle.
When triggered, it enumerates all session files, loads and parses them,
and then checks their expiration.
This becomes too CPU-intensive, especially on low-power devices like Raspberry Pi.
It's unlikely that you actually need to compact every second over the long time.
This change adds a configurable throttling for idle-compaction.
Default behavior is unchanged.
