[Netty 5] AttributeMap removal

We want to remove the AttributeMap and also the attr(...) method from the Channel / ChannelHandlerContext.

There are two possibilities for this:

• provide another way to map things to a Channel. How exactly this is done is not clear yet, but perhaps the extensibility improvements will allow for wrapping the Channel, or state can be kept in a ChannelHandler.
• Just don’t offer a replacement and have the user use stateful ChannelHandler.

[trustin]

I find the ability to attach an arbitrary object to a Channel very useful in practice, especially when I do not wish to introduce tight coupling between handlers or when there’s stateful information that do not belong to any handlers. The AttributeMap API is somewhat ugly although it serves my needs well.

[normanmaurer]

@johnou I am not convinced that this is the right thing to use for every read. For every read you should store the state directly in the ChannelHandler IMHO.