On server side errors (5xx) dump headers

[cstamas]

New feature, improvement proposal

To ease debugging, in case of server side errors (5xx), Resolver should provide option to dump full request and full server response (w/ headers, no content).

This should be done in 1.9.x and 2.x.

[cstamas]

Unsure: allegedly, this would "help Cloudflare" to track down reasons for often seen HTTP 50x responses.