Enable checkstyle rule that looks for final classes.

Enabled the checkstyle rule that checks if classes that could
be final are made final, and fixed the violations that were
reported.

Change-Id: I07ca27f420855116959c1bef3648330cd266ddd8
diff --git a/src/main/java/net/onrc/onos/core/datastore/hazelcast/HZClient.java b/src/main/java/net/onrc/onos/core/datastore/hazelcast/HZClient.java
index 4cc68b4..32ff15b 100644
--- a/src/main/java/net/onrc/onos/core/datastore/hazelcast/HZClient.java
+++ b/src/main/java/net/onrc/onos/core/datastore/hazelcast/HZClient.java
@@ -30,7 +30,7 @@
 import com.hazelcast.core.HazelcastInstance;
 import com.hazelcast.core.IMap;
 
-public class HZClient implements IKVClient {
+public final class HZClient implements IKVClient {
     private static final Logger log = LoggerFactory.getLogger(HZClient.class);
 
     static final long VERSION_NONEXISTENT = 0L;