Tighten HEADERS and DATA frame order requirements.

[bencebeky]

• Clarify HTTP message definition.
• Require the connection to be closed upon receipt of frames in
  incorrect order.
• Refer to HTTP messages in push stream section.

This addresses #2858.

[MikeBishop]

This text was already somewhat lacking by not discussing the possibility of extension frames being interleaved; stating that the push stream consists of "responses" rather than "frames" extends this shortcoming to the push stream section.

I'm open to fixing this in a separate PR, if you prefer, though.

[bencebeky]

This text was already somewhat lacking by not discussing the possibility of extension frames being interleaved; stating that the push stream consists of "responses" rather than "frames" extends this shortcoming to the push stream section.

I'm open to fixing this in a separate PR, if you prefer, though.

Good catch. I added a paragraph to explicitly allow unknown frames to the message exchange section. I also modified the wording in the push section to be more permissive of interleaved frames ("by" instead of "consists of").

[LPardue]

LGTM