Reads the latest client startup record for a single client.

def ReadClientStartupInfo(self, client_id, cursor=None):
    """Reads the latest client startup record for a single client."""
    query = (
        "SELECT startup_info, UNIX_TIMESTAMP(timestamp) "
        "FROM clients, client_startup_history "
        "WHERE clients.last_startup_timestamp=client_startup_history.timestamp "
        "AND clients.client_id=client_startup_history.client_id "
        "AND clients.client_id=%s")
    cursor.execute(query, [db_utils.ClientIDToInt(client_id)])
    row = cursor.fetchone()
    if row is None:
      return None

    startup_info, timestamp = row
    res = rdf_client.StartupInfo.FromSerializedString(startup_info)
    res.timestamp = mysql_utils.TimestampToRDFDatetime(timestamp)
    return res