* Extend the existing bidirectional cancellation thread to a three-thread case.
* Also create a three-thread case where the events actually depend on each other in a cycle.

The second new test made me realize a bigger problem: Cancellation really needs to destroy the PromiseNode in the remote thread synchronously before acknowledging, because the destructors of that PromiseNode could access objects captured from the requesting thread. So, I went ahead and fixed that.