provide a bound on the total amount of data stored in S3

[sbrudenell]

title.

I'll have to think about what the implementation should be. If we exceed a threshold, we could delete older backups, or keep "sparser" backups (e.g. delete every 2nd backup of the smallest timeframe).

I could also see a use case where the primary knob is how much data to store in S3, and we somehow autoselect a preservation policy. That will be a separate issue.