Secondary Certs: Open PRs

There are two open PRs in my old repo which I didn't merge before publishing the first WG draft simply so that we could adopt the draft everyone had read without sneaking changes in.  Both need a little polishing to incorporate feedback before merging.  Once that's done, I'm planning to merge them soon and they'll appear in -01 unless someone screams.  Fair warning.

First, #17<https://github.com/MikeBishop/http2-certs/pull/17> implements Kazuho's suggestion to remove the AUTOMATIC_USE flag and  replace it with pre-emptively sending USE_CERTIFICATE.  This is generally good, though it further emphasizes the problem discussed in a moment.  Second, #18<https://github.com/MikeBishop/http2-certs/pull/18> tracks the changes in Exported Authenticator #9<https://github.com/tlswg/tls-exported-authenticator/pull/9>, so I'll merge after Nick does.

There's still the open issue that some of the frames get sent on stream that are half-closed or closed, which is odd in HTTP/2 and downright impossible in HTTP/QUIC.  That probably means moving them to Stream 0, but other creative ideas are welcome.