express: Implement a graceful shutdown handling `forever stop`

server.js

/*
 Run server by forever with `--killSignal=SIGTERM`:
 
 ```
 $ forever --killSignal=SIGTERM start ./server.js
 ```

 Now `gracefulShutdown` function is called when you stop the server by `forever stop`:

 ```
 $ forever stop
 ```
*/
const express = require('express');
app = express();

//...

const server = app.listen(3000);

//
// Graceful shutdown:
//
const gracefulShutdown = function() {
  const ms = 2000;
  logger.debug(`[server] gracefulShutdown: shutdown after ${ms} ms...`);
  setTimeout(() => {
    logger.debug(`[server] gracefulShutdown: aborting...`);
    process.abort();
  }, ms);
};
process.on('SIGTERM', gracefulShutdown); // for kill
process.on('SIGINT', gracefulShutdown); // for Ctrl+C

module.exports = app;