rgw: adjust versioning enable/suspend api

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: fix access to object through the null instance

Beforehand we were following olh if the explicit 'null' instance was
specified.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
cls_rgw: inc olh epoch when updating log

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw, cls_rgw: fix update of olh to reflect non existing object

need to update olh appropriately when removing the last reference of an
object.

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