Added kryo serializer for ImmutableByteSequence

Serialization was failing when the inner ByteBuffer instance was a
HeapByteBuffer. Also, simplified ImmutableByteSequence.toString()

Change-Id: I4f75086a9b6536205fb43b78a0e4d2bfce9e8a5c
diff --git a/core/store/serializers/src/main/java/org/onosproject/store/serializers/KryoNamespaces.java b/core/store/serializers/src/main/java/org/onosproject/store/serializers/KryoNamespaces.java
index 113ad3d..e51ee78 100644
--- a/core/store/serializers/src/main/java/org/onosproject/store/serializers/KryoNamespaces.java
+++ b/core/store/serializers/src/main/java/org/onosproject/store/serializers/KryoNamespaces.java
@@ -537,7 +537,7 @@
             )
             .register(ClosedOpenRange.class)
             .register(DiscreteResourceCodec.class)
-            .register(ImmutableByteSequence.class)
+            .register(new ImmutableByteSequenceSerializer(), ImmutableByteSequence.class)
             .build("API");