rgw: some more manifest work

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: manifest iterator work

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: a bumch of manifest related changes and fixes

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