Added --until-empty option for cli delete.

By default, delete --recursive will perform one pass at deleting all
objects and containers; so if objects revert to previous versions or if
new objects or containers otherwise arise during the process, the
account or container(s) may not be empty once done.

The until-empty option activates multiple passes to keep trying to fully
empty and delete the containers or account. Note that this mode could
run forever if something else is making new items faster than they’re
being deleted.