Clean up archived data

[timbru]

The migrations leave archived directories behind. These are useful if a downgrade is needed, but they can use a lot of space over time.

Downgrades are typically unnecessary (or that's the intention). More importantly: they can only be done shortly after an upgrade. Otherwise, too many changes will be lost.

So, it's good to delete old archived migration data after a while. Users can do this manually, but Krill could also do this automatically.

I am leaning toward an automated process with an option for users to turn this off (opt-out) in configuration. Input is welcome.

[timbru]

Done as part of Support migrations using non-disk storage #1094