Gulp does not wait for gulpfile.js to complete when SIGTERM is sent to gulp-cli

### What were you expecting to happen?

When I click CTRL+C, a sigterm is sent to gulp. I have created a watcher which spawns a child_process when anything under `src/**` is triggered. to ensure that the instance has fully exited before we restart a new instance. My instance has some async cleanup (~50 ms). I've added an `exitHandle` for SIGINT and SIGTERM handler to my gulpfile.js to prevent the process from exiting immediately. When this completes properly, there are some console logs, and then the user should be given the terminal after a short delay.

### What actually happened?

Gulpfile completes, but gulp-cli has exited. This results in losing the current process losing the terminal input. (eg `KleptoKat@Computer:~/app$`) with the console outputs after this line. It requires an extra enter keypress which is not a huge deal but it's a little annoying.

### Please give us a sample of your gulpfile

Here is a smaller gulpfile:

```js
const gulp = require('gulp');
const { spawn } = require('child_process');
const { EventEmitter } = require('events');

let instance;
let instanceRunning = false;
let instanceChanged = new EventEmitter();
let watchCb;

function startAll(cb) {
instance = spawn('node', [ '-e', '"setTimeout(() => {}, 20000)"'], {
stdio: [undefined, process.stdout, process.stderr],
env: process.env,
detached: true,
});
instanceRunning = true;
instanceChanged.emit('started');

instance.addListener('exit', (code) => {
console.log(`Instance exited with code ${ code }`);
instanceRunning = false;
instance = undefined;
instanceChanged.emit('stopped');

if (code === 10) {
startAll();
}
})

if (cb) { cb() };
}

function kill(cb) {
if (instance) {
console.log(`Killing instance...`);
const onStopped = () => {
instanceChanged.removeListener('stopped', onStopped);
cb();
}

instanceChanged.addListener('stopped', onStopped);

instance.kill();
} else {
cb();
}
}

function watch(cb) {
// TODO: restart individual services w/hot reload :O
gulp.watch('src/**/*', gulp.series(
'build',
'kill',
'startAll'
));

watchCb = cb;
}

gulp.task('kill', kill);
gulp.task('startAll', startAll);
gulp.task('build', buildSrc);
gulp.task('watch', watch);
gulp.task('default', gulp.series(
'build',
'startAll',
'watch',
'kill'
));

function exitHandle(signal) {
console.log('Exit handle', signal. instanceRunning);

if (watchCb) {
watchCb();
} else {
process.exit(0);
}
setTimeout(() => {}, 50000);

if (instance) {
instance.kill();
} else {

}
}

process.on('SIGINT', exitHandle);
process.on('SIGTERM', exitHandle);
process.on('SIGHUP', exitHandle);
```

### Terminal output / screenshots

When CTRL C is entered:
```sh
INFO SERVER: Gateway started on port 1337
^CExit handle undefined
[18:09:20] Finished 'watch' after 2 min
[18:09:20] Starting 'kill'...
Killing instance...
INFO BrokerManager: Stopping

KleptoKat@Computer:~/app$ INFO broker: ServiceBroker is stopped. Good bye.
INFO BrokerManager: stopped
Instance exited with code 0
[18:09:20] Finished 'kill' after 26 ms
[18:09:20] Finished 'default' after 2.1 min
[ ENTER KEY IS PRESSED HERE ]
KleptoKat@Computer:~/app$
```

### Please provide the following information:
* OS & version [e.g. MacOS Catalina 10.15.4]: Ubuntu 20
* node version (run `node -v`): v10.16.3
* npm version (run `npm -v`): 6.9.0
* gulp version (run `gulp -v`): CLI 2.2, local 4.0.2

I can help provide a more replicable environment if need be. It's just slightly annoying. A possible fix may be to add an argument, like --waitForGulpfileToExit which will enable this kind of waiting behaviour.