Merge the attribute map of ChannelHandlerContext into Channel

- Fixes #2136

transport/src/main/java/io/netty/channel/DefaultChannelHandlerContext.java

@@ -17,14 +17,15 @@ package io.netty.channel;
 
 import io.netty.buffer.ByteBufAllocator;
 import io.netty.channel.ChannelHandler.Skip;
-import io.netty.util.DefaultAttributeMap;
+import io.netty.util.Attribute;
+import io.netty.util.AttributeKey;
 import io.netty.util.concurrent.EventExecutor;
 import io.netty.util.internal.PlatformDependent;
 
 import java.net.SocketAddress;
 import java.util.WeakHashMap;
 
-final class DefaultChannelHandlerContext extends DefaultAttributeMap implements ChannelHandlerContext {
+final class DefaultChannelHandlerContext implements ChannelHandlerContext {
 
     // This class keeps an integer member field 'skipFlags' whose each bit tells if the corresponding handler method
     // is annotated with @Skip. 'skipFlags' is retrieved in runtime via the reflection API and is cached.
@@ -279,6 +280,11 @@ final class DefaultChannelHandlerContext extends DefaultAttributeMap implements
         return name;
     }
 
+    @Override
+    public <T> Attribute<T> attr(AttributeKey<T> key) {
+        return channel.attr(key);
+    }
+
     @Override
     public ChannelHandlerContext fireChannelRegistered() {
         DefaultChannelHandlerContext next = findContextInbound(MASK_CHANNEL_REGISTERED);