rgw: admin: introduce objects expire-stale list and rm

rgw: admin: introduce objects expire-stale list and rm

These commands help cleanup stale expired objects after a reshard has happened
Contains the following changesets:

* rgw_bucket: add a AdminOp function to cleanup stale expired objects
There is also a dry run option, which will allow listing to just view the
objects before we actually delete them
* rgw_bucket: introduce rgw_object_get_attr helper function
Since this could be re-used across other functions that try to get a single
xattr, both get acl and the new object expire stale commands now use this
function to read the value of a single xattr

Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>