Update to Spring 5

[diredev]

Spring 5 and SpringBoot 2 have been out for a while already. Try updating the project to use the new framework.

This includes:

• Update dependencies, clean up
• Replacing the Checker Framework with Spring's own annotation(?)
• Replace Jetty with an even smaller container(?)
• Try the new Reactive APIs

[diredev]

Update was fairly straight-forward. Everything seems to work as expected.

This still uses the servlet based approach, no reactive stuff here. See #7.