An implementation of IScheduler that doesn't actually delay. This is useful for tests
which aren't interested in precise timing, but just want to be able to use a scheduler.
For more fine-grained control, use FakeScheduler.
public Task Delay(TimeSpan delay, CancellationToken cancellationToken)
Returns a task that will complete "nearly immediately": there's no set delay, but the task
yields before completing, avoiding synchronous execution leading to subtle bugs. The cancellation
token passed in is observed, causing an exception if it's already canceled.
