code: @Nonnull
    public static Deferred deferred(final String executor, final Route.OneArgHandler handler) {
      return new Deferred(executor, (req, deferred) -> {
        try {
          deferred.resolve(handler.handle(req));
        } catch (Throwable x) {
          deferred.reject(x);
        }
      });
    }
docs: Functional version of {@link Deferred#Deferred(Initializer)}. To use ideally with one
  or more {@link Executor}:
  
  <pre>{@code
  {
  executor("cached", Executors.newCachedExecutor());
  
  get("/fork", deferred("cached", req -> {
  return req.param("value").value();
  }));
  }
  }</pre>
  
  This handler automatically {@link Deferred#resolve(Object)} or
  {@link Deferred#reject(Throwable)} a route handler response.
  
  @param executor Executor to run the deferred.
  @param handler Application block.
  @return A new deferred handler.