[SPARK-15121] Improve logging of external shuffle handler

## What changes were proposed in this pull request?

Add more informative logging in the external shuffle service to aid in debugging who is connecting to the YARN Nodemanager when the external shuffle service runs under it.

## How was this patch tested?

Ran and saw logs coming out in log file.

Author: Thomas Graves <tgraves@staydecay.corp.gq1.yahoo.com>

Closes #12900 from tgravescs/SPARK-15121.

common/network-shuffle/src/main/java/org/apache/spark/network/shuffle/ExternalShuffleBlockHandler.java

@@ -35,6 +35,7 @@ import org.apache.spark.network.server.RpcHandler;
 import org.apache.spark.network.server.StreamManager;
 import org.apache.spark.network.shuffle.ExternalShuffleBlockResolver.AppExecId;
 import org.apache.spark.network.shuffle.protocol.*;
+import org.apache.spark.network.util.NettyUtils;
 import org.apache.spark.network.util.TransportConf;
 
 
@@ -86,7 +87,8 @@ public class ExternalShuffleBlockHandler extends RpcHandler {
         blocks.add(blockManager.getBlockData(msg.appId, msg.execId, blockId));
       }
       long streamId = streamManager.registerStream(client.getClientId(), blocks.iterator());
-      logger.trace("Registered streamId {} with {} buffers", streamId, msg.blockIds.length);
+      logger.trace("Registered streamId {} with {} buffers for client {} from host {}", streamId, 
+        msg.blockIds.length, client.getClientId(), NettyUtils.getRemoteAddress(client.getChannel()));
       callback.onSuccess(new StreamHandle(streamId, msg.blockIds.length).toByteBuffer());
 
     } else if (msgObj instanceof RegisterExecutor) {