CancellableContinuation should not be a Job

[elizarov]

CancellableContinuation interface should cease to extend Job interface. Job is too "heavy weight". Job has a concept of "children" and "completing" state where it waits for its children to complete. None of that is applicable to CancellableContinuation. It has much simpler state-machine. Thus, Job has many methods and extensions that are not truly applicable to CancellableContinuation.

From implementation standpoint, CancellableContinuation has to extend JobSupport for all that Job machinery which is not needed. This complicates the code itself and makes it harder to performance-optimize CancellableContinuation to its specific use-cases.

CancellableContinuation will still keep many methods from the Job, notably invokeOnCompletion, cancel and isActive/isCompleted/isCancelled.