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.
It's probably worth changing the supertype interface, seems low risk and eliminates the need for a weird explanation.