Add draft of database upgrade code for version 3:
- Delete any duplicate OwnIdentity objects created by the restoreOwnIdentity bug https://bugs.freenetproject.org/view.php?id=6085
- Restore the OwnIdentitys afterwards
- NOT ready for deployment yet: It will throw NPE because there is no IdentityFetcher yet when we call upgradeDB(). This will require the following issue to be fixed first: https://bugs.freenetproject.org/view.php?id=6004