HBASE-27810. Check if the executor has already been shut down before submitting new job (#5207)

Signed-off-by: Wellington Chevreuil <wchevreuil@apache.org>
Signed-off-by: Duo Zhang <zhangduo@apache.org>
(cherry picked from commit a6e89bf624)

hbase-zookeeper/src/main/java/org/apache/hadoop/hbase/zookeeper/ZKWatcher.java

@@ -604,8 +604,10 @@ public class ZKWatcher implements Watcher, Abortable, Closeable {
     LOG.debug(prefix("Received ZooKeeper Event, " + "type=" + event.getType() + ", " + "state="
       + event.getState() + ", " + "path=" + event.getPath()));
     final String spanName = ZKWatcher.class.getSimpleName() + "-" + identifier;
+    if (!zkEventProcessor.isShutdown()) {
       zkEventProcessor.execute(TraceUtil.tracedRunnable(() -> processEvent(event), spanName));
     }
+  }
 
   // Connection management