[RuntimeAsync] Avoid keeping continuation state alive after resumption

[jakobbotsch]

As suggested here: dotnet/runtimelab#3063 (comment)

We can either ensure DispatchContinuations does not keep the continuation object itself alive, or we can null out the arrays of the continuation object during resumption. The latter is more robust but comes with code size/resumption cost.

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.