[SPARK-14254][CORE] Add logs to help investigate the network performance

## What changes were proposed in this pull request?

It would be very helpful for network performance investigation if we log the time spent on connecting and resolving host.

## How was this patch tested?

Jenkins unit tests.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #12046 from zsxwing/connection-time.

common/network-common/src/main/java/org/apache/spark/network/client/TransportClientFactory.java

@@ -123,7 +123,10 @@ public class TransportClientFactory implements Closeable {
   public TransportClient createClient(String remoteHost, int remotePort) throws IOException {
     // Get connection from the connection pool first.
     // If it is not found or not active, create a new one.
+    long preResolveHost = System.nanoTime();
     final InetSocketAddress address = new InetSocketAddress(remoteHost, remotePort);
+    long hostResolveTimeMs = (System.nanoTime() - preResolveHost) / 1000000;
+    logger.info("Spent {} ms to resolve {}", hostResolveTimeMs, address);
 
     // Create the ClientPool if we don't have it yet.
     ClientPool clientPool = connectionPool.get(address);
@@ -235,7 +238,7 @@ public class TransportClientFactory implements Closeable {
     }
     long postBootstrap = System.nanoTime();
 
-    logger.debug("Successfully created connection to {} after {} ms ({} ms spent in bootstraps)",
+    logger.info("Successfully created connection to {} after {} ms ({} ms spent in bootstraps)",
       address, (postBootstrap - preConnect) / 1000000, (postBootstrap - preBootstrap) / 1000000);
 
     return client;