Serve: send back 431 error instead of panicking when request header too big

[dkegel-fastly]

This is a POC that's just good enough to pass a smoke test, presented for discussion.

TODO:

• verify that this is the right approach
• on total header length too big, send 413
• on URI too long, send 414
• add tests verifying behavior when each part of the request from client is too big
• add tests verifying behavior when each part of the response from origin is too big