rgw_file: leaf objects (which store Unix attrs) can be deleted when children exist
We chose to omit this initially in the interest of saving latency, but especially since addition of logic to store Unix attrs on directory leaf objects. The issue has been re-raised by Gui Hecheng here: https://github.com/nfs-ganesha/nfs-ganesha/issues/153
In that ticket I proposed that we could have a low-cost check for children already cached, and a more expensive test that would be performed only if the fast check failed. After discussion w/Frank Filz, I want to withdraw the proposed fast check until such time (if it arrives) as we have a consistent caching option that would prevent false positives.