Get a dictionary of in-progress work units for some work spec.

        The dictionary is from work unit name to work unit definiton.
        Units listed here should be worked on by some worker.

def list_pending_work_units(self, work_spec_name, start=0, limit=None):
        """Get a dictionary of in-progress work units for some work spec.

        The dictionary is from work unit name to work unit definiton.
        Units listed here should be worked on by some worker.

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