http2: Update to h2-14

* Expand frame length field to 24 bits
* Add new SETTINGS:
  - SETTINGS_MAX_FRAME_SIZE
  - SETTINGS_MAX_HEADER_LIST_SIZE
* Update libnghttp2 HPACK
* Remove END_SEGMENT flag

Change-Id: I5906322ad5a4d61c963ed95fada9415e66e146da
Reviewed-on: https://code.wireshark.org/review/3357
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>