WIP: Add background async task runner

[sultaniman]

Hi,

This PR ports background task runner from Responder
and Starlette.

Use case example

from django.utils import background

@background.task
def my_task():
    # Do some work

def hello(request):
    my_task()  # Call task to execute in background

    render(request, template)

This is still WIP PR so please give you reviews so I can adjust things.

Thanks.

[vsag96]

Looks amazing, just a suggestion, why not have the no of workers equal to the number of cores like celery? Thank you for this.

[timgraham]

I don't see why this needs to be part of Django instead of a third-party package. Generally, we don't add anything to django.utils that Django itself doesn't use. Anyway, the first step is to get consensus on our new feature on the django-developers mailing list.

[sultaniman]

Fair points. Thanks for feedback. I believe Django core team should move mailing list into GitHub issues.