Help make port to Jetty 9 easier.

This doesn't solve a problem per-se. This just moves a few things around so that there are fewer changes required to port to Jetty 9. With these changes a port to Jetty 9 looks like https://github.com/tvaughan/ring-jetty9-adapter/commit/e510dda5279f779bc63c67bd7d142cbcc0280eae. (This is actually incomplete. I haven't figured out how to handle the real API breakage between 7 and 9, like http://git.eclipse.org/c/jetty/org.eclipse.jetty.project.git/commit/jetty-util/src/main/java/org/eclipse/jetty/util/thread/QueuedThreadPool.java?id=8e98148350f215e2ec58d0b936ab08ce22229d6a.) Thanks!