Add time period offsets to calculation

[xombiemp]

For each period we want to include the previous period's duration in
the calculation for when to delete snapshots.

For example, if we are saving 24 hours and 3 days, we don't want the
calculation for days to just be 3 days ago from the current time, we
want it to be 3 days ago from 24 hours ago.

To accomplish this, an offset has been added to the formula to include
the duration of the previous periods along with the current period's
duration.

[xombiemp]

Here are the changes you wanted done from my previous pull request. I also moved the deletes and periods definitions outside of the volumes loop so they aren't reinitialized on each volume. I can't see any problem with that... can you?

[xombiemp]

Just wondering if there are other changes you would like to see before this request gets pulled?

[zwily]

Conceptually this looks good, I'd just like to write a couple tests to verify the logic before I merge it. I think you're right about the intended behavior though.