storage: prevent possible re-migration

- wrap each account migration in db transaction/commit/rollback
- don't remove account dbs during the start of a migration
- change the criteria for performing a migration to be missing
  migrated data, instead of the existence of legacy data

Change-Id: I2039deb000a44285c6a227689277d9cb0ba00b24
Reviewed-by: Sébastien Blin <sebastien.blin@savoirfairelinux.com>