Modify handler-fn to support `:async?` option in jetty server.

[baskeboler]

Should look something like this:

(defn handler-fn
  [{:keys [controller-interceptors
           router-interceptors]
    :as   app-config}
   system
   routes]
  (fn handle*
    ([http-request]
     (->
      (xiana/flow->
       (state-build app-config system routes http-request)
       (runner/run router-interceptors route)
       (additional-interceptors controller-interceptors run-controller))
      (xiana/extract)
      (get :response)))
    ([request respond _]
     (respond (handle* request)))))