Ensure ListenNotifications not returning before it started listening

[ramondeklein]

The ListenNotifications is performing two asynchronuous operatations:

1. Register for the actual registration of the notifications.
2. Notifcations that are actually coming in.

Because notifications only are coming in after step 1, you need to make sure that this has been completed, before the function returns.

Go doesn't suffer from this problem, because it doesn't expose async to the caller and it probably uses a callback for the notifications. There are several ways to fix this:

• Use a Go-style approach that implements an async subscribe function that only returns after it actually has subscribed. This function takes an async delegate as a parameter that is invoked whenever a notification is signalled.
• Use a more traditional C# implementation that returns Task<IObservable> and returns the observable after subscribing to the object. Although mixing async and IObservable is non-typical, it makes sense here.

See also #1130 (comment).