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: http://www.varnish-cache.org/svn/trunk/varnish-cache@3322 d4fa192b-c00b-0410-8231-f00ffab90ce4