Get a dictionary of finished work units for some work spec.

        The dictionary is from work unit name to work unit definiton.
        Only work units that have been successfully completed are
        included.

def list_finished_work_units(self, work_spec_name, start=0, limit=None):
        """Get a dictionary of finished work units for some work spec.

        The dictionary is from work unit name to work unit definiton.
        Only work units that have been successfully completed are
        included.

        """
        return self.registry.filter(WORK_UNITS_ + work_spec_name + _FINISHED,
                                    start=start, limit=limit)