Upgrade unfiltered-jetty to jetty 9

Let's do this with the same module name and a major version number increment for unfiltered. If the java 7 requirement is onerous to users of the module, we can resurrect jetty 8 in an `unfiltered-jetty8` module. (Let's wait for complaints before going to that trouble.)

This PR contains changes by Joni, Pasi, and myself.