private SchedulerStateChangeListener createLivenessListener(AtomicReference<List<String>> liveProcessors) {
    return () -> {
      LOG.info("Leader detected change in list of live processors.");
      doOnProcessorChange(liveProcessors.get());
    };
  }
Creates a listener for LivenessCheckScheduler class.
Invoked by the leader when the list of active processors in the system changes.
@return an instance of SchedulerStateChangeListener.
samza-azure/src/main/java/org/apache/samza/coordinator/AzureJobCoordinator.java