There is a potential lock-order inversion between a worker thread and the ban-lurker and there is nothing we can do about it:  They come from opposite ends of the world.
There is a potential lock-order inversion between a worker thread
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: 2c9807fa3ff65b17195bd55dc8a6c4261e10127b@5432 d4fa192b-c00b-0410-8231-f00ffab90ce4

gquintard/Varnish-Cache,varnish/Varnish-Cache,varnish/Varnish-Cache,1HLtd/Varnish-Cache,wikimedia/operations-debs-varnish,alarky/varnish-cache-doc-ja,drwilco/varnish-cache-old,gauthier-delacroix/Varnish-Cache,varnish/Varnish-Cache,wikimedia/operations-debs-varnish,gauthier-delacroix/Varnish-Cache,franciscovg/Varnish-Cache,wikimedia/operations-debs-varnish,zhoualbeart/Varnish-Cache,mrhmouse/Varnish-Cache,zhoualbeart/Varnish-Cache,gauthier-delacroix/Varnish-Cache,chrismoulton/Varnish-Cache,mrhmouse/Varnish-Cache,feld/Varnish-Cache,ssm/pkg-varnish,ajasty-cavium/Varnish-Cache,varnish/Varnish-Cache,drwilco/varnish-cache-drwilco,mrhmouse/Varnish-Cache,ajasty-cavium/Varnish-Cache,1HLtd/Varnish-Cache,ambernetas/varnish-cache,drwilco/varnish-cache-drwilco,mrhmouse/Varnish-Cache,feld/Varnish-Cache,alarky/varnish-cache-doc-ja,franciscovg/Varnish-Cache,franciscovg/Varnish-Cache,ssm/pkg-varnish,drwilco/varnish-cache-drwilco,1HLtd/Varnish-Cache,alarky/varnish-cache-doc-ja,ambernetas/varnish-cache,ssm/pkg-varnish,gquintard/Varnish-Cache,chrismoulton/Varnish-Cache,zhoualbeart/Varnish-Cache,franciscovg/Varnish-Cache,drwilco/varnish-cache-old,franciscovg/Varnish-Cache,alarky/varnish-cache-doc-ja,feld/Varnish-Cache,feld/Varnish-Cache,gauthier-delacroix/Varnish-Cache,gquintard/Varnish-Cache,zhoualbeart/Varnish-Cache,ssm/pkg-varnish,alarky/varnish-cache-doc-ja,wikimedia/operations-debs-varnish,ambernetas/varnish-cache,ajasty-cavium/Varnish-Cache,varnish/Varnish-Cache,ajasty-cavium/Varnish-Cache,1HLtd/Varnish-Cache,feld/Varnish-Cache,gauthier-delacroix/Varnish-Cache,ssm/pkg-varnish,chrismoulton/Varnish-Cache,chrismoulton/Varnish-Cache,wikimedia/operations-debs-varnish,drwilco/varnish-cache-old,ajasty-cavium/Varnish-Cache,gquintard/Varnish-Cache,zhoualbeart/Varnish-Cache,mrhmouse/Varnish-Cache,chrismoulton/Varnish-Cache