Add us_listen_socket_close and graceful shutdown example

examples/GracefulShutdown.js

@@ -0,0 +1,39 @@
+/* Minimal example that shuts down gracefully */
+
+const uWS = require('../dist/uws.js');
+const port = 9001;
+
+/* We store the listen socket here, so that we can shut it down later */
+let listenSocket;
+
+const app = uWS./*SSL*/App({
+  key_file_name: 'misc/key.pem',
+  cert_file_name: 'misc/cert.pem',
+  passphrase: '1234'
+}).get('/shutdown', (res, req) => {
+  if (listenSocket) {
+    res.end('Okay, shutting down now!');
+    /* This function is provided directly by µSockets */
+    uWS.us_listen_socket_close(listenSocket);
+    listenSocket = null;
+  } else {
+    /* We just refuse if alrady shutting down */
+    res.close();
+  }
+}).listen(port, (token) => {
+  /* Save the listen socket for later shut down */
+  listenSocket = token;
+  /* Did we even manage to listen? */
+  if (token) {
+    console.log('Listening to port ' + port);
+
+    /* Stop listening soon */
+    setTimeout(() => {
+      console.log('Shutting down now');
+      uWS.us_listen_socket_close(listenSocket);
+      listenSocket = null;
+    }, 1000);
+  } else {
+    console.log('Failed to listen to port ' + port);
+  }
+});