Server builder refactor

This isn't finished. Still need to

- [ ] Do the same for netty-server. This is just a jetty server builder.
- [ ] Decide on the right migration name/package. I don't like `unfiltered.jetty.refactor` but it works for testing.

Anyway, it passes as many tests as master does (`unfiltered.request.GzipSpecJetty` fails?) so that's good.
reading this pull linearly so I may come back to this but do you think this could cause confusion? I guess as long as it's documented it's okay.