ASGI app implementation

### Description
Django channels implements the ASGI spec. Many other frameworks also implement that spec, and any ASGI-compliant "app" can be *mounted* as part of another ASGI app(though usually framework-specific integrations are created to better exploit framework-specific features).

Recent versions of django channels and Starlette implements [ASGI 3.0](https://asgi.readthedocs.io/en/latest/specs/main.html).

An ASGI implementation should enable integration with django-channels, starlette(and starlette-based frameworks) and any other asgi-based frameworks.