Implement HTTP generic consumers

The HTTP event spec in ASGI is a little complex, and it would be nice to have an Async-capable generic consumer for HTTP to allow easier writing of long-poll applications. Something like:

class LongPoller(AsyncHttpConsumer):
    async def request(self, body):
        await asyncio.sleep(10)
        await self.send_response(200, "foo")

[andrewgodwin]

I believe we even have tests that check that more_body support works in Channels, it’s just not implemented in the server yet. But we want it to work once it is.

[matthiask]

@hishnash Yes. The implementation was not my idea, similar code exists in channels/http.py already: https://github.com/django/channels/blob/6afd423/channels/http.py#L180