ESI includes can hit the waiting list and since the state is stored

ESI includes can hit the waiting list and since the state is stored
on the worker thread stack, we cannot just let another thread pick
up.

This is not as much a fix as a workaround: simply sleep until the
object we wait for is no longer busy.

The issue may have to be revisited in the long run, but for 2.0.x
this will have to do.

Fixes #345

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@3322 d4fa192b-c00b-0410-8231-f00ffab90ce4