[SPARK-21701][CORE] Enable RPC client to use SO_RCVBUF and SO_SNDBUF in SparkConf.

## What changes were proposed in this pull request?

TCP parameters like SO_RCVBUF and SO_SNDBUF can be set in SparkConf, and `org.apache.spark.network.server.TransportServe`r can use those parameters to build server by leveraging netty. But for TransportClientFactory, there is no such way to set those parameters from SparkConf. This could be inconsistent in server and client side when people set parameters in SparkConf. So this PR make RPC client to be enable to use those TCP parameters as well.

## How was this patch tested?

Existing tests.

Author: xu.zhang <[email protected]>

Closes apache#18964 from neoremind/add_client_param.

Author: xu.zhang

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

@@ -210,6 +210,14 @@ private TransportClient createClient(InetSocketAddress address)
       .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, conf.connectionTimeoutMs())
       .option(ChannelOption.ALLOCATOR, pooledAllocator);
 
+    if (conf.receiveBuf() > 0) {
+      bootstrap.option(ChannelOption.SO_RCVBUF, conf.receiveBuf());
+    }
+
+    if (conf.sendBuf() > 0) {
+      bootstrap.option(ChannelOption.SO_SNDBUF, conf.sendBuf());
+    }
+
     final AtomicReference<TransportClient> clientRef = new AtomicReference<>();
     final AtomicReference<Channel> channelRef = new AtomicReference<>();