Eliminating double-muxing

[MikeBishop]

This pull request changes the current document substantially, with the following major points of change:

• The HTTP/2 outer frame containing the stream number is replaced with a (otherwise similar) frame format that doesn't contain a stream number. All stream management is delegated to QUIC.
• HTTP exchanges take up two streams per request/response -- one for data, one for everything else (but mostly headers). This permits retaining the current behavior (also from HTTP/2) of not flow-controlling non-DATA traffic, and avoids double-framing bodies.
• HPACK frames acquire sequence numbers to enable ordering at the decoder. This is decidedly suboptimal, but no worse than current and allows us to keep moving and leave the potential redesign of HPACK for the future.

It may be more useful to look at the final content rather than the diff -- in places, GitHub isn't finding enough of the retained text to make the diff sensible.