re-implement the cache purging routine to
avoid using ioutil.ReadDir which can lead
to high allocations when there are cache
directories with lots of content, or
when cache is installed in memory constrainted
environments.
Instead rely on a callback function where we
are not using memory no-more than 8KiB per
cycle.
Precursor for this change refer #9425, original
issue pointed by Caleb Case <caleb@storj.io>
