Caching the CancellationToken will prevent ObjectDisposedException race condition

[jstnlef]

To be a good citizen, it is important that the caller of this method can (and should) call Dispose() on the returned token source. The problem though, is that each time cts.Token was called before, it would try to create a new CancellationToken which means that it could throw an ObjectDisposedException leading to noisy logs.