Add cache channel implementation

Cache channels are useful for active cache management where a backend
publishes a feed of objects that are to be expired. See
http://ietfreport.isoc.org/idref/draft-nottingham-http-cache-channels/
for an expired internet draft about this.

git-svn-id: http://www.varnish-cache.org/svn/trunk@4512 d4fa192b-c00b-0410-8231-f00ffab90ce4