Addressed Checkstyle and PMD violations in the registry module.

Note some PMD errors can't be cleaned up because the curator API throws
raw Exception on many methods. PMD complains about catching raw Exception.

Change-Id: I974e3e713417d71de8fcee3833d53ab003a71eae
diff --git a/src/main/java/net/onrc/onos/core/registry/IdBlock.java b/src/main/java/net/onrc/onos/core/registry/IdBlock.java
index d905917..7184239 100644
--- a/src/main/java/net/onrc/onos/core/registry/IdBlock.java
+++ b/src/main/java/net/onrc/onos/core/registry/IdBlock.java
@@ -1,9 +1,9 @@
 package net.onrc.onos.core.registry;
 
 public class IdBlock {
-    private long start;
-    private long end;
-    private long size;
+    private final long start;
+    private final long end;
+    private final long size;
 
     public IdBlock(long start, long end, long size) {
         this.start = start;