Graceful shutdown of RingHandler to finish in-flight requests

I'm not at all familiar with http-kit so this could be entirely wrong.

My approach is to shutdown the RingHandler's ExecutorService and wait a number of milliseconds for existing threads to terminate before performing a shutdownNow.

I haven't touched the HttpServer itself, since org.httpkit.server/run-server keeps a reference to the RingHandler and calls close on it first anyway.