[TLS] "Draft Version" extension

[Eric Rescorla <[e-mail]>]

Experience with HTTP 2 has shown that it's convenient to be able to
distinguish which draft version you're implementing so you don't end
worrying about interop between various I-D versions. Accordingly,
for TLS 1.3 I'd like to define a temporary extension that indicates which
TLS 1.3 draft it implements. The idea here is that the client would signal
TLS 1.3 and then "draft 3" and the server would only negotiate 1.3 if
it had a compatible draft. Otherwise it would fall back to TLS 1.2.

My proposed extension would be as follows:

struct {
  uint16 draft_version;
} DraftVersionExtension;

And would have a code point in the (unofficially) experimental range:
0xff02.
We would not need a permanent allocation since once TLS 1.3 ships you
would  send no extension at all.

I've implemented this in NSS and it's trivial to do.

Thoughts?
-Ekr