FEATURE: Use time stamp from kurisu in update process

Old behaviour

When checking to see if a kara need to be updated from kurisu, we checked the time stamp on the local machine with get_mtime. It was slow and not reliable.

New behaviour

The time stamp of the file is cached in the database, so no mtime is needed. It will be better for the disk usage and the time needed to update the database.

NOTE: The repo_timestamp should be defaulted to cached_mtime at upgrade time, it's better than 0 because it will permit to not re-dl all the database at the next update