Lists the clients associated with keywords.

def ListClientsForKeywords(self, keywords, start_time=None):
    """Lists the clients associated with keywords."""
    res = {kw: [] for kw in keywords}
    for kw in keywords:
      for client_id, timestamp in iteritems(self.keywords.get(kw, {})):
        if start_time is not None and timestamp < start_time:
          continue
        res[kw].append(client_id)
    return res