Improve max-worker sync-to-async handling.

#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](https://github.com/django/daphne/blob/6199d509c200cdd5102b39a3461a62984adebaf7/daphne/server.py#L131)).
* 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.

Start in daphne/server.py at the loop-configuration point linked in the issue, then inspect the test added for #422 and the existing CLI entry point. Verify that Server.run() applies the worker limit, that --max-workers is covered by tests, and that the environment-variable path is deprecated without breaking existing behavior.

Written by the indexing model from the issue text.