Add overridable DefaultOptionsProvider.AfterDisconnectAsync() callback

[philon-msft]

Enables graceful release and shutdown in custom DefaultOptionsProviders. For example, Microsoft.Azure.StackExchangeRedis uses a token refresh Timer that should be stopped when the connection is closed.

This approach using a callback was suggested as an alternative to previous PR #2922

[NickCraver]

In the refresh timer, why don't we check if any of the weak refs to multiplexers are still alive instead? And if not, stop the timer. This would be a lot less race-prone and ambiguous/confusing (e.g. disconnects happen all the time, per-server). Happy to chat more when back next week!

[philon-msft]

In the refresh timer, why don't we check if any of the weak refs to multiplexers are still alive instead? And if not, stop the timer. This would be a lot less race-prone and ambiguous/confusing (e.g. disconnects happen all the time, per-server). Happy to chat more when back next week!

Yes that works better - I'll remove a muxer from my list if its weak ref goes bad or it throws an ObjectDisposedException, and if the list is empty I'll stop the timer.