synchronized Optional<Instant> activeUntil(String singletonId) {
        return Optional.ofNullable(janitors.get(singletonId)).map(janitor -> janitor.doom.get());
    }
The instant until which this container holds the exclusive lease for activation of singletons with this ID.
The container may abandon the lease early, if deactivation is triggered and completes before the deadline.
Unless connection to the underlying ZK cluster is lost, the returned value will regularly move forwards in time.
zkfacade/src/main/java/com/yahoo/vespa/curator/SingletonManager.java