SETTINGS parsing assumes fixed-length fields

[LPardue]

Discussion on https://github.com/quicwg/base-drafts/issues/1556 highlighted that there is a problem with our parsing of some SETTINGS parameters as defined in draft 09 (and 13, current at time of writing).

The offending line is https://github.com/bbc/nghq/blob/master/lib/frame_parser.c#L235

We should observe how the HTTP/QUIC issue is resolved before committing to a fix.