Yield client urns.

def GetInput(self):
    """Yield client urns."""
    client_list = GetAllClients(token=self.token)
    logging.debug("Got %d clients", len(client_list))
    for client_group in collection.Batch(client_list, self.client_chunksize):
      for fd in aff4.FACTORY.MultiOpen(
          client_group,
          mode="r",
          aff4_type=aff4_grr.VFSGRRClient,
          token=self.token):
        if isinstance(fd, aff4_grr.VFSGRRClient):
          # Skip if older than max_age
          oldest_time = (time.time() - self.max_age) * 1e6
        if fd.Get(aff4_grr.VFSGRRClient.SchemaCls.PING) >= oldest_time:
          yield fd