Logic for combining and splitting repeated headers
Define the BLOCKED frame.
Implement h2-12 reworking of PRIORITY.
Define COMPRESSED flag.
Add support for DATA frame compression.
Turn off hostname checking in test mode.
Add COMPRESS_DATA setting.

Advertise the fact that we accept compressed data frames. Right now we
don't police that this setting is obeyed, but that's OK.
We support draft 12 now.