Lists the clients associated with keywords.

def ListClientsForKeywords(self, keywords, start_time=None, cursor=None):
    """Lists the clients associated with keywords."""
    keywords = set(keywords)
    hash_to_kw = {mysql_utils.Hash(kw): kw for kw in keywords}
    result = {kw: [] for kw in keywords}

    query = """
      SELECT keyword_hash, client_id
      FROM client_keywords
      FORCE INDEX (client_index_by_keyword_hash)
      WHERE keyword_hash IN ({})
    """.format(", ".join(["%s"] * len(result)))
    args = list(iterkeys(hash_to_kw))
    if start_time:
      query += " AND timestamp >= FROM_UNIXTIME(%s)"
      args.append(mysql_utils.RDFDatetimeToTimestamp(start_time))
    cursor.execute(query, args)

    for kw_hash, cid in cursor.fetchall():
      result[hash_to_kw[kw_hash]].append(db_utils.IntToClientID(cid))
    return result