Provide a way to implement an ChannelInbound/OutboundMessageHandler conveniently without extending an adapter class

- Add ChannelHandlerUtil and move the core logic of ChannelInbound/OutboundMessageHandler to ChannelHandlerUtil
- Add ChannelHandlerUtil.SingleInbound/OutboundMessageHandler and make ChannelInbound/OutboundMessageHandlerAdapter implement them.  This is a backward incompatible change because it forces all handler methods to be public (was protected previously)
- Fixes: #1119