CompletableFuture<?> register(SingletonWorker singleton) {
            Task task = Task.register(singleton);
            tasks.offer(task);
            return task.future;
        }
Attempt to activate or deactivate if status has changed.
If activation fails, we release the lock, so a different container may acquire it.
zkfacade/src/main/java/com/yahoo/vespa/curator/SingletonManager.java