Add tiered backup retention feature.

[MetalJulia]

To elaborate, instead of just keeping a fixed number of backups, such a feature would keep X backups per frequency.
As an example:

• Four thirty-minute backups are kept
• Four two-hourly backups are kept for the period before those
• After that, four six-hourly backups are kept.

Such a configuration is just an example of how such a thing could work. Perhaps a way it could be implemented would be to export a clean state (I'm not sure how pulling backups of a running MC server even works) of the world, then feed it into a light external backup utility to avoid re-inventing the wheel.

[AnterCreeper]

I need, too!

[gigabit101]

Fixed in 1.0.19