Add http2 support

[sergeyfrolov]

Issue moved from the old repo. FWIW, http2 is more common now, and is worth looking into again.

We might want to have this support because

• All the modern browsers advertise http2 first in ALPN, so, if we want to parrot them and advertise http2 as well, we will get an increasing amount of decoy servers responding "yep, let's talk http2". Then the http/1.1-style incomplete request would trigger "wtf did you just send that's no http2" response.

• http2 might have some desirable features that we can use

• Per http2 specification https://http2.github.io/http2-spec/#FrameHeader :

  Implementations MUST ignore and discard any frame that has a type that is unknown.

  This might allow us to ignore the upload limit and substantially improve performance.