Version negotiation

[martinthomson]

@tfpauly identified a pattern that is useful for version negotiation: rather than build a mechanism that won't be used until v2, hoist the negotiation out of the protocol to the next layer down. That way you can take advantage of the extension mechanisms at that layer seeing more use (and therefore gaining use-it-or-lose it advantages).

Add a new section specifically about version negotiation in the design principles section.

See also IPv6 Ethertype, ALPN, intarchboard/program-edm#8

It might also be worth pointing to https://eprint.iacr.org/2016/072