Migration test

@philips @xiangli-cmu This pull request adds the migration tests from v1 to v2. One tests a single instance upgrade and the other tests a cluster upgrade.

There's also one bug fixed in `store.Store` where entries read from disk are parsing the expiration date using the TZ which causes problems when comparing to `store.Permanent`. I added a minimum time that ensures that expiration times can't be before that.