rgw_file: implement rgw_setattr

rgw_file: implement rgw_setattr

Introduce a new RGWSetattrs RGWOp descendant, to create or replace
sets of attrs on buckets or objects.

This version of the change uses the standard RGWRADOS::set_attrs op
(we want attribute changes to (e.g.) sync with other changes).

Previous versions of this changed incorrectly masked the values
of st->st_ino in RGWFileHandle::stat(), now fixed.

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>
(cherry picked from commit 4de1c3c260265f821ebee842d49cb35bf49d8e4e)

Fixes: http://tracker.ceph.com/issues/17311
Fixes: http://tracker.ceph.com/issues/17332