public boolean isClosed() {
        return (!sessionOpen || watcher.getState().equals(Watcher.Event.KeeperState.Expired));
    }
If this is called, we assume we're in shutdown situation, or we are doing it because we need a new session.
Thus we only need to free up resources, no need to notify anyone.
clustercontroller-core/src/main/java/com/yahoo/vespa/clustercontroller/core/database/ZooKeeperDatabase.java