the response, not the time of the request.  If the document was not in
cache and the backend took a long time to respond, the document would
end up with a negative computed age, which when printed with %u would
appear as a large positive number (a few seconds short of either 2^32
or 2^64 depending on the size of time_t).

git-svn-id: http://www.varnish-cache.org/svn/trunk@1142 d4fa192b-c00b-0410-8231-f00ffab90ce4