Say you have a middleware (`middleware.js`) that looks like this:
```js
export async default function middleware(req, ctx) {
ctx.waitUntil(tooLongFunction())
}
async function tooLongFunction() {
console.log('tooLongFunction started')
await new Promise((resolve) => setTimeout(resolve, 10000))
console.log('tooLongFunction finished')
}
```
When you run this middleware locally with `vercel dev`, you won't see the `tooLongFunction finished` message because the server process is killed right after finishing the response. While Edge Runtime's `server.close()`, which awaits all `waitUntil` promises, is called, but `exit-hook`'s callback in CLI doesn't wait for the `close` promise to resolve.
This PR fixes this by allowing an optional graceful shutdown callback instead of killing the process immediately.
