Guard against trying to migrate database to the current version

For my analysis of why this should be necessary, please see #28

The third commit was written because I wanted to test the migration in two steps: from v1 to v2 and then from v2 to v3 It is a very hacky and could certainly be improved. The main thing is however, that it didn't actually fail in the same way that my application did (see calls!505 (comment 1375591) for more information).

The last commit is the one guarding against a migration to the version the database is already in.