Fix a thread leak

Change-Id: I6a39abd634c2be684cf567c1f2b2e43d17f46fc3
diff --git a/app/src/main/java/org/onosproject/segmentrouting/SegmentRoutingManager.java b/app/src/main/java/org/onosproject/segmentrouting/SegmentRoutingManager.java
index 0ab5ef0..00bfe93 100644
--- a/app/src/main/java/org/onosproject/segmentrouting/SegmentRoutingManager.java
+++ b/app/src/main/java/org/onosproject/segmentrouting/SegmentRoutingManager.java
@@ -496,6 +496,7 @@
         processor = null;
         linkListener = null;
         deviceListener = null;
+        groupHandlerMap.forEach((k, v) -> v.shutdown());
         groupHandlerMap.clear();
 
         dsNextObjStore.destroy();