nearline-storage: shutdown ScheduledExecutor on shutdown

Motivation:
The AbstractBlockingNearlineStorage should stop all executor services on
shutdown.

Modification:
Shutdown ScheduledExecutor on AbstractBlockingNearlineStorage#shutdown

Result:
clean shutdown.

Acked-by: Albert Rossi
Acked-by: Lea Morschel
Target: master
Require-book: no
Require-notes: no

modules/dcache/src/main/java/org/dcache/pool/nearline/script/ScriptNearlineStorage.java

@@ -199,6 +199,7 @@ public void shutdown()
         flushExecutor.shutdown();
         stageExecutor.shutdown();
         removeExecutor.shutdown();
+        scheduledExecutor.shutdown();
         executor.shutdown();
     }