Return most recent client from list of clients.

def GetMostRecentClient(client_list, token=None):
  """Return most recent client from list of clients."""
  last = rdfvalue.RDFDatetime(0)
  client_urn = None
  for client in aff4.FACTORY.MultiOpen(client_list, token=token):
    client_last = client.Get(client.Schema.LAST)
    if client_last > last:
      last = client_last
      client_urn = client.urn
  return client_urn