Enhancement: Parallel upload/copy of backups

[MaxJa4]

Here lies an opportunity for adding a real world feature on top of this refactoring: as copying is a read only operation, we could now run all of the operations in parallel and considerably speed up execution times for people that back up to multiple backends. Do you think we should add this here?

In case you're interested, here's an example how it can be done: https://github.com/offen/docker-volume-backup/blob/b441cf3e2b1273462de59bcabbf0c81a6061525f/cmd/backup/exec.go#L146-L169

Originally posted by @m90 in https://github.com/offen/docker-volume-backup/pull/135#discussion_r933421835

[MaxJa4]

Just to keep track and not forget that!