Fix combination callbacks - #1887
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UseCallbacks combined the callbacks with +=. Invoking a multicast Task returning delegate returns only the last target's Task, so when UseCallbacks was called more than once, for example by an extension package plus user code, every earlier callback ran unawaited: its exceptions unobserved and its async work racing the combination method. Extensions.Then documents the same hazard for Func<Task>. They are now held in lists and awaited in order. The no callback path still returns Task.CompletedTask without allocating.
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UseCallbacks combined the callbacks with +=. Invoking a multicast Task returning
delegate returns only the last target's Task, so when UseCallbacks was called
more than once, for example by an extension package plus user code, every
earlier callback ran unawaited: its exceptions unobserved and its async work
racing the combination method. Extensions.Then documents the same hazard for
Func.
They are now held in lists and awaited in order. The no callback path still
returns Task.CompletedTask without allocating.