The current backend implementation reads the headers all at once, as a
big buffer, then manually chops them up, and later on, in the startfetch
step, Varnish loops through all the headers and prints them.

This is inconvenient for custom backends that are most likely going to
use http_SetH() (directly or via http_SetHeader(), http_PrinfHeader() or
others), which also prints the headers being added. As a result, those
implementations end up logging the header twice.

To work around the issue we can push the burden of logging the beresp
headers onto the backend implementation. It does change one test, as
now the Timestamp:Beresp log record appears after the headers instead
of before.