Find a way to fix broken differential backups

[rzvoncek]

Project board link

It might happen that a differential backup looses a file in the storage, but this file is still referenced in the manifest, and hopefully present on the backed up node. If that's the case, we could make the nodetool verify not just detect the inconsistencies, but also fix them. This can happen either by re-uploading the file, or looking for it somewhere else in the bucket (which is probably harder to do than say).