Drop the consistency check for health-changes, at the cost of a
slight bias[1] for hosts before, or after, then one that changed health,
according to how its health changed.

Count all failures to pick against the retry count.

Fixes #361


git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@3359 d4fa192b-c00b-0410-8231-f00ffab90ce4