HBASE-20102 AssignmentManager#shutdown doesn't shut down scheduled executor

Signed-off-by: Josh Elser <elserj@apache.org>

hbase-server/src/main/java/org/apache/hadoop/hbase/master/AssignmentManager.java

@@ -3790,6 +3790,7 @@ public class AssignmentManager extends ZooKeeperListener {
 
     // Shutdown the threadpool executor service
     threadPoolExecutorService.shutdownNow();
+    scheduledThreadPoolExecutor.shutdownNow();
     zkEventWorkers.shutdownNow();
     regionStateStore.stop();
   }