implement Nth-timespan full backups

[sbrudenell]

instead of having the largest timeframe in the policy always have full backups, we could do a full backup every N of the largest timeframe.

for example, a preservation policy of 6m could keep a full backup every 2 months, and an incremental backup every other month.

for most (all?) purposes, this looks like just defining one more timeframe, that isn't one of the predefined ones.