Refactor fixity-related methods

The commits below can (and should) be squashed before merging. They've been separated out for understanding all the moving parts, but are probably unable to stand alone.

Most of this work is in preparation for Modeshape 4 / Infinispan 6, which significantly change some of the APIs we've been using (for the better). This is an attempt to refactor the existing code in a way that will work well enough with both what we have now and prepare us for Modeshape 4.

One controversial change might be  962458b - removing self-healing from fixity failures. The way we're currently doing it is not very good, and has no chance of working going forward.