OwnIdentity: Override upgradeDatabaseFormatVersion5WithoutCommit()

Notice: The WebOfTrust.upgradeDatabaseFormatVersion5() which is already
linked by the added JavaDoc will be added by following commits.

This completes the refactoring of (Own)Identity to not store FreenetURI
as String anymore.
The remaining classes will be dealt with in following commits.
WebOfTrust.upgradeDatabaseFormatVersion3(): Compile-fix for removal of
FreenetURI.removeFrom()
WebOfTrust: Bump database format version, add upgrade code

Not tested yet.

This completes the refactoring to make WOT not store FreenetURI objects
in the database anymore. The upgrade code converts all stored FreenetURI
String.
Also, it completes the whole of adapting WOT to compile against
purge-db4o :)