WaitAsync forceAsync flag

[timcassell]

Currently, for optimization purposes, if it is currently executing on the same context that was provided to WaitAsync or Run or Progress, the continuation will be invoked synchronously. This may not be desired in some instances if the user wants to force an action to run on a separate thread for increased parallelization. For that we can add an optional forceAsync flag to WaitAsync, Run, and Progress functions that, if true, will always schedule the continuation on the desired context. The default will be false.