synchronized void unregistered(String singletonId, SingletonWorker singleton) {
        registrations.remove(singleton);
        if (count.merge(singletonId, -1, Integer::sum) > 0) return;
        count.remove(singletonId);
        janitors.remove(singletonId).shutdown();
    }
Manages {@link com.yahoo.vespa.curator.api.VespaCurator.SingletonWorker}.

@author jonmv
zkfacade/src/main/java/com/yahoo/vespa/curator/SingletonManager.java