DiffBackupManager optimizations

[Birnendampf]

ranging from most to least feasible, I found a couple of places where that manager can be optimized

Backup Creation

- [ ] rather than unlinking files in filter_diff, add them to a list of files that will simply be ignored when recompressing. though the benefit of this is questionable, since those files will eventually be deleted by shutil.rmtree() anyways once the Temporary directory containing them is removed.

[Birnendampf]

consider using zip instead of tar files

the rust zip crate is not in a great state. apart from that, zip is also not a great format, with the way it handles timestamps. will keep this on the backlog for now. it also remains to be seen how much the backup efficiency is degraded by zip and if a compression dictionary could be used to remedy this

[Birnendampf]

and if we are already considering individual file compression, might as well be a bit more like restic. or git, for that matter. this would also conveniently avoid having to compress the entire world with every backup AND it would open up the doors to an "off site, backing up to remote" type of feature