Hack the CancellationToken!!!

_source == null means that the CancellationToken cannot be cancelled
(that is what the property CanBeCancelled checks... so it also means
IsCancellationRequested = false and Register = useless.

Futhermore, WaitHandle checks if the source is disposed, so I'll use it
as a workaround to check if the source is disposed without being able to
reach the source. CnacellationTokenHelper just does that.