Duplex Task<T>

[RxDave]

Implement duplex support for Task<T> similar to IObservable<T>.

Primary use case:
When closing over a local method for duplex invocation, the server blocks until the client returns. This is inefficient; however, if you define an async method instead (that returns Task or Task<T>, but not void), then the query will fail because it would try to serialize the Task result to the server rather than installing a proxy, like it does for IObservable<T> closures.