netty: use composite cumulator for benchmarks

Author: sergiitk

netty/src/main/java/io/grpc/netty/GrpcHttp2ConnectionHandler.java

@@ -32,6 +32,10 @@
 @Internal
 public abstract class GrpcHttp2ConnectionHandler extends Http2ConnectionHandler {
 
+  {
+    setCumulator(COMPOSITE_CUMULATOR);
+  }
+
   @Nullable
   protected final ChannelPromise channelUnused;