Reduce our snapshot cost by a factor of 4.

We don't need 64 snapshots of toby data.  16 is plenty: the last 8
days, 1 per week for the 4 weeks prior to that, and 1 per month for
the 4 months prior to that.  Basically we've got coverage for a week,
a quarter, and a half-year.

Auditors: chris

Test Plan:
None