rgw: rename cls_rgw_obj::key to cls_rgw_obj::loc

to avoid confusion.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
cls_rgw: change data structures to keep single object key structure

instead of keeping the key name and instance separately. Also, translate
key structure into the index key string representation.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: adapt to new objclass interface

still need some more work here

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>