Function krill::commons::util::file::backup_dir

pub fn backup_dir(
    base_path: &PathBuf, 
    target_path: &PathBuf
) -> Result<(), Error>

Recursively copy a base_path (if it's a dir that is), and preserve the permissions timestamps and all that goodness..

This is needed when making a back-up copy when we need to do upgrades on data, which could in theory fail, in which case we want to leave teh old data in place.