Remove memory copy when extract frame in LengthFieldBasedFrameDecoder

Motivation:

We are currently doing a memory cop to extract the frame in LengthFieldBasedFrameDecoder which can be eliminated.

Modifications:

Use buffer.slice(...).retain() to eliminate the memory copy.

Result:

Better performance.

codec/src/main/java/io/netty/handler/codec/LengthFieldBasedFrameDecoder.java

@@ -491,9 +491,7 @@ public class LengthFieldBasedFrameDecoder extends ByteToMessageDecoder {
      * is overridden to avoid memory copy.
      */
     protected ByteBuf extractFrame(ChannelHandlerContext ctx, ByteBuf buffer, int index, int length) {
-        ByteBuf frame = ctx.alloc().buffer(length);
-        frame.writeBytes(buffer, index, length);
-        return frame;
+        return buffer.slice(index, length).retain();
     }
 
     private void fail(long frameLength) {