and the ban-lurker and there is nothing we can do about it:  They
come from opposite ends of the world.

Resolve this by using a TryLock in the ban-lurker and abandon the
attempt if we fail to get the lock.

Fixes:	#796


git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5432 d4fa192b-c00b-0410-8231-f00ffab90ce4