Frame Length Restrictions

I put together a pull request to replace the frame length restriction with
an HTTP application layer restriction that I would like the working group
to consider.

https://github.com/http2/http2-spec/pull/456

Of particular note is that this is in contrast to a previous decision made
by the working group (albeit by coin toss in Seattle) which can be found

https://github.com/http2/http2-spec/issues/260

With the addition of padding to the framing layer, I believe it is
preferable to implement the frame length requirement at the HTTP layer to
allow intermediaries to pad frames without running into frame length
restrictions.

Thanks!
- Jeff