Registering extensions

[Ian Swett <[e-mail]>]

There are two PRs out proposing two different models of extensions.

How extensions are registered is largely orthogonal, so I'd like to break
that off into a separate discussion.

I propose three spaces:
 1) Smallest(ie: 0-255) extensions are allocated by the IETF WG
 2) Medium(ie: 256-65535) extensions are registered with IANA
 3) The rest(ie: 65536+) are registered on a Wiki.

This allows room for real experimentation with a very low bar on
developers, which is ideal from a software and transport development
perspective.

I'm open to any reasonable split of the space into three parts, and I'm
open to 4 bytes or a 62 bit varint, but I do think we should have >=4 bytes
worth of extensions.