rgw: copy_obj uses req_id as tag

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw-admin: bucket list also specifies object namespace

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw-admin: add object stat command

for retrieving object metadata.

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