Dor::Processable#status refactor

This is a minor refactoring of the status method in Dor::Processable.  The logic for calculating version and status code are broken out into a new method, as are the mappings for status to display text, and the old status method is now just a wrapper that calls the new method (the old method still returns what it used to, to maintain compatibility with existing callers).  Also did a bit of cleanup to get rid of superfluous variable initializations in the old logic.