Move QPACK control instructions to dedicated streams

[afrind]

1. Give QPACK encoders and decoders their own unidirectional streams (hardcoded to 6,7,10,11 for now)
2. Define simple framing for encoder stream
3. Move HEADER_ACK from HTTP over QUIC frame to QPACK instruction
4. Introduce Table State Synchronize instruction

Fixes #1121
Fixes #1122

[martinthomson]

Rather than hard-code, I would greatly prefer that this used a stream header.

[afrind]

Can you explain the basis of your preference? A client has no other unidirectional streams defined in HTTP over QUIC, and a server must process/acknowledge at least one header block before opening any push streams. It is further very unlikely the server would open a push stream without first encoding a header block that inserts at least one header to the dynamic table. So in practice it's going to work out to these streams anyways.

How do you see the encode/decode streams as materially different from the control stream, which is hardcoded?

Aside: it occurs to me that a server could compress a PUSH_PROMISE a lot better if it compressed against the client's table, which of course doesn't work ;)

[MikeBishop]

Looks reasonable. The source of the stream header is, I think, the discussion in #910 about eliminating references to particular stream IDs in the application layer. You can make a QUIC interface that depends only on receiving a stream and reading the bytes to figure out what to do with it, rather than needing to open / listen on particular streams by ID.

I think that model is important for things that could crop up mid-connection, but I don't feel as strongly about the header for streams that are unique and persistent.

[MikeBishop]

BTW, I think you might have meant #1122 rather than #1120. #1120 is another PR that has already been merged.

[MikeBishop]

Rebased onto the changes in master, but kept the changes from the original PR.

[MikeBishop]

We should probably specify that these need to be complete instructions (i.e. no truncation), and require an error if one of the length-prefixed blocks ends mid-instruction.