Fields 6 and 7 are reserved for work in progress, so the multiport lane
adverts move up to 9 and 10. Both are still single-byte tags, so the
encoded payload size does not change.
Also emit the lane fields after CertVersion so MarshalPayload stays in
ascending field-number order, matching what protoc-gen-go would produce
and keeping the hand-built expectations in payload_test.go
straightforward.
Field numbers are the only thing that identifies a lane advert, so a node
on this build and one on the previous numbering will each skip the other's
adverts as unknown fields and fall back to a single vanilla tunnel.
Wade Simmons