Returns a generator of all labels in the store.

        If ``content_id`` is set, will restrict results to labels
        containing the provided ``content_id``.

def everything(self, content_id=None):
        '''Returns a generator of all labels in the store.

        If ``content_id`` is set, will restrict results to labels
        containing the provided ``content_id``.
        '''
        if content_id is not None:
            ranges = [((content_id,), (content_id,))]
        else:
            ranges = []

        labels = self.kvl.scan(self.TABLE, *ranges)
        labels = imap(lambda p: self._label_from_kvlayer(*p), labels)
        return labels