Improve max-worker sync-to-async handling.

[carltongibson]

422 ports the old ASGI_THREADS logic from asgiref. (C.f. #319)

Improvements:

• Move the initialisation of the default executor into Server.run() (when configuring the loop).
• Adjust the test from #422 to set up the Server instance, so that's put into effect.
• Add a --max-workers CLI flag, with tests etc., to apply this.
• Deprecate the env var approach.