Drop SETTINGS persistence as discussed at the interim
Specify how to handle PING frames of lengths that aren't 8.

Specifically calls them out as connection errors of type
PROTOCOL_ERROR.
Refactoring of DATA and HEADERS frame as discussed in Layering TF.

Move definition of FINAL flag to DATA and HEADERS frames.
Replace HEADERS+PRIORITY with HEADERS and a separate PRIORITY flag.
Renumber other associated flags now that FINAL is frame-specific.
Add a MSG_DONE flag for future extensibility.
Merge branch 'settingspersist' of github.com:jasnell/http2-spec into layering
Merge branch 'PING' of https://github.com/willchan/http2-spec into layering
Merging HEADER and DATA updates per Jeff Pinner's pull request...
Editorial: explanation of optional priority value in HEADERS
Associated only