Updates existing user notification objects.

def UpdateUserNotifications(self,
                              username,
                              timestamps,
                              state=None,
                              cursor=None):
    """Updates existing user notification objects."""

    query = ("UPDATE user_notification "
             "SET notification_state = %s "
             "WHERE username_hash = %s"
             "AND UNIX_TIMESTAMP(timestamp) IN {}").format(
                 mysql_utils.Placeholders(len(timestamps)))

    args = [
        int(state),
        mysql_utils.Hash(username),
    ] + [mysql_utils.RDFDatetimeToTimestamp(t) for t in timestamps]
    cursor.execute(query, args)