Add request and response infomation in the debug log

---

Fixes #64

*Motivation*

Currently, kop protocol handler only prints header at debug
statements. We also need request and response.

*Modifications*

- Add more request and response info

src/main/java/io/streamnative/kop/KafkaCommandDecoder.java

@@ -142,9 +142,9 @@ public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception
 
         try (KafkaHeaderAndRequest kafkaHeaderAndRequest = byteBufToRequest(buffer, remoteAddress)){
             if (log.isDebugEnabled()) {
-                log.debug("[{}] Received kafka cmd {}",
+                log.debug("[{}] Received kafka cmd {}, the request content is: {}",
                     ctx.channel() != null ? ctx.channel().remoteAddress() : "Null channel",
-                    kafkaHeaderAndRequest.getHeader());
+                    kafkaHeaderAndRequest.getHeader(), kafkaHeaderAndRequest);
             }
 
             switch (kafkaHeaderAndRequest.getHeader().apiKey()) {
@@ -232,8 +232,11 @@ protected void writeAndFlushResponseToClient(Channel channel) {
             try {
                 ResponseAndRequest pair = response.join();
                 if (log.isDebugEnabled()) {
-                    log.debug("Write kafka cmd response back to client. request: {}",
-                        pair.getRequest().getHeader());
+                    log.debug("Write kafka cmd response back to client. \n" +
+                            "\trequest content: {} \n" +
+                            "\tresponse content: {}",
+                        pair.getRequest().toString(),
+                        pair.getResponse().toString(pair.getRequest().getRequest().version()));
                 }
 
                 ByteBuf result = responseToByteBuf(pair.getResponse(), pair.getRequest());