Draft 14 framer stream

Updates for framer, stream, and connection.

Unfortunately, tests fail between these two commits.
I recommend squash-before-merge (so that git bisect works).

If you find commits are too large, please tell me so.  I'll work harder to make commits smaller.
Latest draft defines a fairly complex priority control.
See https://tools.ietf.org/html/draft-ietf-httpbis-http2-14#section-5.3 .
We currently have no prioritization among streams.
I think we want to implement at least weighted priority queue.