Fix ObjectDisposedException thrown in Cache.CleanupLoop

[AnyhowStep]

Resolves #2

We store a copy of the CancellationToken in the Cache's ctor.
Then, we use that inside of CleanupLoop(), and never reference _cts directly.

This way, we avoid having ObjectDisposedException thrown when Dispose() is invoked before IsCancellationRequested check and Task.Delay().