Data Pruning: Prune by round interval

[winder]

Followup to #1096

Problem

The "once" / "daily" configuration does not provide very much control.

Solution

The frequency setting should be changed to a round interval, which would be used to delete data at startup, and every N rounds after.

The setting would behave as follow:

• 0: no pruning.
• N: prune at startup and every N rounds.

There is no more "once" option, if that was desired users would need to update their settings to 0.