rgw: implement listing stale instances from a reshard

rgw: implement listing stale instances from a reshard

Dynamic resharding used to leave behind stale bucket instances; walk through the
metadata pool and identify these instances by comparing the reshard status. If
the reshard status is done, these instances are ok to be cleared. For reshard
status of none we compare against the bucket entry point to ensure that we don't
match the current entry point.

Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>
(cherry picked from commit 0c35a6fab250a817b2752f319b5c9f1da13bb3df)