* storagenode: remove datetime calls in favor of UTC
datetime only has second level granularity whereas string
comparisons don't. Since we're wiping everything anyway, it's
easier to just use UTC everywhere rather than migrate to
datetime calls.
* add utcdb to check that arguments are utc
* storagenodedb: add trivial tests to ensure calls work
This at least tests that all of the timestamps passed in are
in the UTC timezone.
* fix truncated comment and change migrations to be UTC
- Drops some unused indexes
- Applies a computed index to timestamp columns
- Applies a partial index for expired pieces
- Uses BETWEEN to avoid some datetime calls
- Filters expired piece search by those that aren't NULL
