PartitionManager: Shutdown background thread during deactivate

Fixes ONOS-1168.

Change-Id: I7fc4fa3787b552d958e148b96e6a6a9c51291ddb

@@ -86,6 +86,8 @@ public class PartitionManager implements PartitionService {
 
     @Deactivate
     public void deactivate() {
+        executor.shutdownNow();
+
         leadershipService.removeListener(leaderListener);
         clusterService.removeListener(clusterListener);
     }