Deduplicate auth requests

[d-fischer]

AuthProvider instances sometimes get many requests at once in production workloads. That causes them to request a lot of tokens at once.

This can possibly result in:

• spamming id.twitch.tv with requests
• in extreme cases, saving a token that is getting invalidated by too many other tokens being created
• opening many dialog boxes (e.g. Electron)

We aim to get rid of this problem by only sending a single request and reusing that request for responding to all other calls as well.

Definitely consider what happens with requests that require new scopes after the request has already been sent - it might require some sort of scope queue.