Allow `access_token_factory` to accept a coroutine

[milesizzo]

Would it be possible to allow access_token_factory to also accept a coroutine?

If the team is not against it, I'm happy to figure it out and submit a PR.

[droserasprout]

Sure, you're welcome!

[milesizzo]

I made the change then realised that I need the access token to be generated during negotiation (as well as handshake) for our use-case. I figure this is probably a breaking change so decided not to submit it.

I'll construct a change that makes access_token_factory optionally a coroutine only, if that's still useful for others.