Don't use the "used" field to signal suicide for worker threads,
use the "wrq" field which is much more natural.

Set the "used" field to NAN before doing anything and assert that
somebody updated during the task.



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@1685 d4fa192b-c00b-0410-8231-f00ffab90ce4