Make a label from a kvlayer row.

def _label_from_kvlayer(self, key, val):
        '''Make a label from a kvlayer row.
        '''
        (content_id1, content_id2, annotator_id,
         inverted_epoch_ticks) = key
        epoch_ticks = time_complement(inverted_epoch_ticks)

        rel_strength, meta = cbor.loads(val)
        return RelationLabel(content_id1, content_id2, annotator_id,
                             RelationStrength(rel_strength),
                             epoch_ticks=epoch_ticks, meta=meta)