HBCK throws RejectedExecutionException when closing ZooKeeper resources

HBCK throws RejectedExecutionException at the end of run, because the order of closing ZooKeeper resources has been swapped in HBASE-27426.

In ZKWatcher.java close() method first it shuts down the zkEventProcessor and when it fully shut down, it closes the RecoverableZooKeeper (the ZK client). The watcher receives the close event which cannot be submitted to the event processor and throws exception.

I think we need to check whether the executor is able to receive jobs before submitting.