Change Kryo duplicate registration warning to DEBUG message

Change-Id: I30923fb9e8650c0073ba1ff9d50353cb004538cc
diff --git a/utils/misc/src/main/java/org/onlab/util/KryoNamespace.java b/utils/misc/src/main/java/org/onlab/util/KryoNamespace.java
index afd6978..4dbe86e 100644
--- a/utils/misc/src/main/java/org/onlab/util/KryoNamespace.java
+++ b/utils/misc/src/main/java/org/onlab/util/KryoNamespace.java
@@ -512,7 +512,7 @@
                 r = kryo.register(type, serializer, id);
             }
             if (r.getId() != id) {
-                log.warn("{}: {} already registered as {}. Skipping {}.",
+                log.debug("{}: {} already registered as {}. Skipping {}.",
                         friendlyName(), r.getType(), r.getId(), id);
             }
             log.trace("{} registered as {}", r.getType(), r.getId());