Enhancement: Add code to detect old cache directories

We've added code to detect old cache directories of repositories that
haven't been used in a long time, restic now prints a note when it detects
that such dirs exist. Also, the option `--cleanup-cache` was added to
automatically remove such directories. That's not a problem because the
cache will be rebuild once a repo is accessed again.

https://github.com/restic/restic/pull/1436
