Return a (deferred) cached Koji user name for this change.

def user(self):
        """ Return a (deferred) cached Koji user name for this change. """
        # Note, do any tasks really have an "owner_id", or are they all
        # "owner"?
        owner_id = getattr(self.task, 'owner_id', self.task.owner)
        return self.task.connection.cache.user_name(owner_id)