With Vary, Prefetch and degraded mode, a session sleeps not on a

With Vary, Prefetch and degraded mode, a session sleeps not on a
particular object, because we cannot know beforehand if it will work out
for us, but sleeps on any one of potentially multiple busy objects becoming
ready for us to test against.

Therefore it makes sense to move the waiting list from the object to the
object head, as this both simplifies the code and eliminates a refhold on
busy objects.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@2389 d4fa192b-c00b-0410-8231-f00ffab90ce4