Try to improve RTR version negotiation.

Try to improve RTR version negotiation.

RFC8210 and especially draft-ietf-sidrops-8210bis are underspecified when
it comes to inital version negotiation. The authors seem to have a very
different view on how this works compared to the various RTR cache
implementations.

Reducing the version on any session close is a problem since it often leads
to downgraded sessions where not needed. Instead require the server to send
PDUs with their correct version (either a code 4 error, a cache response
or cache reset pdu).

Extensively tested against various modes of StayRTR.
Also tested against routinator which is currently not following the spec
(https://github.com/NLnetLabs/routinator/issues/919) and breaks on unknown
versions.

This is probably not the last change to make RTR version negotiation work
but it is a step in the right direction.

OK tb@