Code cleanup
Use a small segment size so the persistence test runs quickly

For some reason, creating a 100MB instead of a 2k file adds 6s to the
test execution time.
Don't use allocate on-disk storage when running the bench suite

It adds 6s startup time.

Also, as an unfortunate side effect, the resources for these benchmarks
are initialized as part of our standard test suite, even though they're
not used.
Use time.Time to represent times, instead of ints

Our interval files are now more intuitively named as well.
Use time.Duration
Make the migrator tests run faster