Shutdown to gain a callback version

[ktoso]

Our shutdown is async yet we only expose a blocking version of wait() on the Shutdown object.

    public struct Shutdown {
        private let receptacle: BlockingReceptacle<Void>

        init(receptacle: BlockingReceptacle<Void>) {
            self.receptacle = receptacle
        }

        public func wait(atMost timeout: TimeAmount) throws {
            guard self.receptacle.wait(atMost: timeout) != nil else {
                throw TimeoutError(message: "Shutdown did not complete", timeout: timeout)
            }
        }

        public func wait() {
            self.receptacle.wait()
        }
    }

Users may want to just provide a callback to run on completion of the shutdown.

NIO's API shape is

func shutdownGracefully(queue: DispatchQueue, _ callback: @escaping (Error?) -> Void)

So something similar could be good.

We assume the global queue currently, so we perhaps should rather ask to be provided the queue to use as well.

[ktoso]

Thx @tomerd for feedback