private String zLog(String logMessage) {
    return String.format("[Processor-%s] %s", processorIdStr, logMessage);
  }
Verify that the predecessor still exists. This step is needed because the ZkClient subscribes for data changes
on the path, even if the path doesn't exist. Since we are using Ephemeral Sequential nodes, if the path doesn't
exist during subscription, it is not going to get created in the future.
samza-core/src/main/java/org/apache/samza/zk/ZkLeaderElector.java