synchronized CompletableFuture<?> unregister(SingletonWorker singleton) {
        String id = registrations.remove(singleton);
        if (id == null) throw new IllegalArgumentException(singleton + " is not registered");
        return janitors.get(id).unregister(singleton)
                       .whenComplete((__, ___) -> unregistered(id, singleton));
    }
Manages {@link com.yahoo.vespa.curator.api.VespaCurator.SingletonWorker}.

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