Test scheduling - stack dive

[lmagyar]

We should check to not make stack dives!!!
TaskCompletionSource-s for operations are created without TaskCreationOptions.RunContinuationsAsynchronously, and nearly all ContinueWith uses TaskContinuationOptions.ExecuteSynchronously.
When we go from sync OnNotifyUnhandledException and OnNotifyPaused to TAP async equivalents, there are two await Task.Yield(), these should break most of the loops.
NOTE: SendResponse's TaskCompletionSource.Task should continue ASAP, and not wait for the other activities in the WF, this way WF can run on the tail of the operations!
See eg.: http://stackoverflow.com/questions/28321457/taskcontinuationoptions-runcontinuationsasynchronously-and-stack-dives