Added support for replicating device offline topology events: Added missing files

+package org.onlab.onos.store.device.impl;
+
+import org.onlab.onos.net.DeviceId;
+import org.onlab.onos.store.Timestamp;
+
+/**
+ * Information published by GossipDeviceStore to notify peers of a device
+ * going offline.
+ */
+public class InternalDeviceOfflineEvent {
+
+    private final DeviceId deviceId;
+    private final Timestamp timestamp;
+
+    /**
+     * Creates a InternalDeviceOfflineEvent.
+     * @param deviceId identifier of device going offline.
+     * @param timestamp timestamp of when the device went offline.
+     */
+    public InternalDeviceOfflineEvent(DeviceId deviceId, Timestamp timestamp) {
+        this.deviceId = deviceId;
+        this.timestamp = timestamp;
+    }
+
+    public DeviceId deviceId() {
+        return deviceId;
+    }
+
+    public Timestamp timestamp() {
+        return timestamp;
+    }
+
+    // for serializer
+    @SuppressWarnings("unused")
+    private InternalDeviceOfflineEvent() {
+        deviceId = null;
+        timestamp = null;
+    }
+}

+package org.onlab.onos.store.device.impl;
+
+import org.onlab.onos.net.DeviceId;
+import org.onlab.onos.store.Timestamp;
+
+import com.esotericsoftware.kryo.Kryo;
+import com.esotericsoftware.kryo.Serializer;
+import com.esotericsoftware.kryo.io.Input;
+import com.esotericsoftware.kryo.io.Output;
+
+/**
+ * Kryo Serializer for {@link InternalDeviceOfflineEvent}.
+ */
+public class InternalDeviceOfflineEventSerializer extends Serializer<InternalDeviceOfflineEvent> {
+
+    /**
+     * Creates a serializer for {@link InternalDeviceOfflineEvent}.
+     */
+    public InternalDeviceOfflineEventSerializer() {
+        // does not accept null
+        super(false);
+    }
+
+    @Override
+    public void write(Kryo kryo, Output output, InternalDeviceOfflineEvent event) {
+        kryo.writeClassAndObject(output, event.deviceId());
+        kryo.writeClassAndObject(output, event.timestamp());
+    }
+
+    @Override
+    public InternalDeviceOfflineEvent read(Kryo kryo, Input input,
+                               Class<InternalDeviceOfflineEvent> type) {
+        DeviceId deviceId = (DeviceId) kryo.readClassAndObject(input);
+        Timestamp timestamp = (Timestamp) kryo.readClassAndObject(input);
+
+        return new InternalDeviceOfflineEvent(deviceId, timestamp);
+    }
+}