DistributedFlowRuleStore: register FlowRuleEvent

Change-Id: I1f904a283c8e619a6ec9094d62ddfba44a41c119

@@ -158,6 +158,7 @@ public class DistributedFlowRuleStore
             serializerPool = KryoNamespace.newBuilder()
                     .register(DistributedStoreSerializers.STORE_COMMON)
                     .nextId(DistributedStoreSerializers.STORE_CUSTOM_BEGIN)
+                    .register(FlowRuleEvent.class)
                     .build();
         }
     };