[quicwg/base-drafts] GREASE the flags byte (#311)

[ianswett <[e-mail]>]

Google-QUIC had an issue where post-handshake QUIC packets were being dropped, because a bit changed in the public flags, and a middlebox no longer identified it as QUIC.

In order to ensure the entire public flags byte is usable, I suggest we GREASE it by defining the entire space as valid.

Building on Jana's proposal for the public header with a flags byte as a set of codepoints: https://gist.github.com/janaiyengar/b05acb3af17d6e938d25befc69c11eae

The goal is to provide a simple mathematical mapping from any value of the public flags onto a frame type.  

My suggestion is to lookup the frame type, and if it's not defined, mod the flags value by the number of frame types defined in that phase(short or long form) and pick the type with that index when ordering the frame types by value.

If an initial packet number is chosen randomly, and the connection ID is as well, greasing the flags byte and version (#112 and #296) covers everything that is not encrypted.

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