Rename worker->used to worker->lastused and make it an optimization to
set it from a convenient timestamp along the way.

If it is not set when the thread goes on the unemployment queue, expend
a timestamp on setting it.



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@3182 d4fa192b-c00b-0410-8231-f00ffab90ce4