Comment 14 for bug 660307

OK I think we can start by always backing up while doing an upgrade from n -> n+1.
I don't know what a gzipped ttl file is so why not just copy the activity.sqlite to another back_activity.sqlite?
After upgrade is done we need to check the DB if its properly built. If not we try to recover from the backup DB and upgrade again.
I also like the idea of setting the schema to -1 in case of a kill.