Since now routers use discovery instead of maintaining their own ping handling, we need to handle the case when discovery is turned off or we're dealing with a router which is pre-MR. To do that we need to change the discovery behavior when discovery is turned off locally or when the peer has discovery off or no MR at all. Currently in those cases discovery turns into a no-op. With this change we will make discovery behave as a standard ping. The response doesn't change the local peer view.