reduce log level (#216)

Author: gfukushima

src/main/java/org/ethereum/beacon/discovery/pipeline/PipelineImpl.java

@@ -35,7 +35,7 @@ public synchronized Pipeline build() {
                 try {
                   handler.handle(envelope);
                 } catch (Throwable t) {
-                  LOG.warn(
+                  LOG.debug(
                       "Unexpected error in pipeline handler {}",
                       handler.getClass().getSimpleName(),
                       t);

src/main/java/org/ethereum/beacon/discovery/pipeline/handler/HandshakeMessagePacketHandler.java

@@ -152,7 +152,7 @@ public void handle(Envelope envelope) {
           ex);
       markHandshakeAsFailed(envelope, session);
     } catch (Throwable t) {
-      LOG.warn(
+      LOG.debug(
           "Unexpected error while processing handshake [{}] from node {}",
           packet,
           session.getNodeRecord(),