rgw: better api for metadata mutation

rgw: better api for metadata mutation

Unlike the regular user and bucket metadata that are plain
raw rados objects, the otp info is controlled by object
class operations. The metadata manager mutate operation
will deal with the metadata related work that is needed
to happen (objv_tracker, update meta log), and call to
the operation that modifies the otp info.

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