Reads the user labels for a list of clients.

def MultiReadClientLabels(self, client_ids):
    """Reads the user labels for a list of clients."""
    res = {}
    for client_id in client_ids:
      res[client_id] = []
      owner_dict = self.labels.get(client_id, {})
      for owner, labels in iteritems(owner_dict):
        for l in labels:
          res[client_id].append(rdf_objects.ClientLabel(owner=owner, name=l))
      res[client_id].sort(key=lambda label: (label.owner, label.name))
    return res