When during a lookup we encounter a busy object, queue the session on

When during a lookup we encounter a busy object, queue the session on
the objects waitinglist and disembark the worker thread so it can do
something sensible in the mean time.

This feature is unimportant in normal operation, but crucial to
resource management if a popular URL suddenly takes a long time to
reply from the backend.

Without this bit if semi-nasty code, we would tie up one worker
thread per client while waiting for the backend to come to it's
senses.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@478 d4fa192b-c00b-0410-8231-f00ffab90ce4