rgw: clean up system obj interfaces

Create a new structure to use, RGWRados::SystemObject, similar to the
previous changes made for the data objects.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: s/RGWRados::ObjectCtx/RGWObjectCtx

Can't forward declare nested class.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: adjust to new interfaces

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