Introduce ChannelHandlerInvoker, dedeciated for invoking event handler methods, and move most handler invocation code in ChannelHandlerContext to the default ChannelHandlerInvoker implementation

- Add ChannelHandlerInvoker and its default implementation
- Add pipeline manipulation methods that accept ChannelHandlerInvoker
- Rename Channel(Inbound|Outbound)Invoker to
  Channel(Inbound|Outbound)Ops to avoid confusion
- Remove the Javadoc references to the package-private interfaces