Response and HttpResponseBodyPart should return ByteBuf

[dgolombek]

Similar to #1951 , Response and HttpResponseBodyPart should optionally return ByteBufs instead of just ByteBuffer or byte[], in case the caller will be using the response data with Netty (or just cares about performance). This be much more efficient if the LazyResponseBodyPart factory is used instead of the eager variant obviously. LazyResponseBodyPart already has getBuf(), but that doesn't help when you have a full Response, and it's not part of the interface. I'll submit a patch for this as well.

[dgolombek]

@hyperxpro anything I can do to help get this reviewed and merged? I have some work time to help with this project, starting with a few things we need, but then can also help with backlog of bugs or whatever else makes sense. Thanks.