Default ServerConfig implementations do not shut down gracefully

At the moment, several of the server backends will terminate in-progress connections instead of shutting down gracefully. Wherever possible, we should make the default to do a graceful shutdown - this might not be available given the underlying server APIs available in each.

Need to write a test in ServerContract and handle each backend in turn to see if we can make it pass.

[daviddenton]

I’d ideally like to see all of the server’s do graceful shutdown by default (unless it makes them slow to stop if there are no requests). Each of the ServerConfigs should not take any extra config (to make things simple), but also be structured in a way that people can implement their own versions with minimum effort - eg. reusing the Servlet in the Jetty, or the handler in Undertow.

[nlochschmidt]

@daviddenton I’ve done a preliminary pull request to make stop() block until inflight requests are handled. I’ve chosen 10 seconds as the default graceful shutdown timeout, but the shutdown will occur as soon as the request is handled. This works for small responses but I haven’t checked for streaming and for really large responses where sending the response takes a long time.