Clean up netty native packaging

There are multiple native libraries available for netty, but we cannot seem to get them working in Karaf. Figure out a way around this, so that we provide a full-features experience.

One way of achieving this would be to shade native libraries, so they end up in a single jar. This feels like something that should be done upstream – and in fact they go a step further with netty-all.

This gets simpler with netty-4.1.74, as the native libraries are fragments which attach to class-bearing artifacts, solving a lot of the issues.