Jetty9 API Changes

These are the changes to the Jetty9 Webservice API that are being made to make this service consistent with the API of the web-routing service. 
Another option for this would be to have `get-server-context` treat a nil server-id as meaning to pull the `:default` context.  This way, you could just pass along whatever `(:server-id opts)` returns to `get-server-context` without the `add-*` functions having to know anything about a `:default` context.