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>