WIP: Test where asgi app is spammed with requests and the context (response headers) is always correct

[tomwojcik]

@hhamana I tested your PR. It works fine. I'm worried about regressions where context is not cleared though.

I'd like to add a test where the client is spamming asgi app with requests. In the meantime, half of them fails. If we can check if response headers are correct after that, it should prove there are no regressions in real world apps with huge traffic.

The only thing left is to figure out why await asyncio.sleep is not working.