Netty4: switch to composite cumulator (#49478)

The default merge cumulator used in netty transport leads to additional
GC pressure and memory copying when a message that exceeds the chunk
size is handled. This is especially a problem on G1 GC, since we get
many "humongous" allocations and that can in theory cause real memory
circuit breaker to break unnecessarily.

modules/transport-netty4/src/main/java/org/elasticsearch/transport/netty4/Netty4SizeHeaderFrameDecoder.java

@@ -32,6 +32,10 @@ final class Netty4SizeHeaderFrameDecoder extends ByteToMessageDecoder {
 
     private static final int HEADER_SIZE = TcpHeader.MARKER_BYTES_SIZE + TcpHeader.MESSAGE_LENGTH_SIZE;
 
+    {
+        setCumulator(COMPOSITE_CUMULATOR);
+    }
+
     @Override
     protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
         try {