Add custom completion handler queue

[maxep]

Allow dispatching callback on a different queue that .main to further allow getting Clock.now from a different thread.

See thread in #85

[Reflejo]

Sorry, what's the point of this change again? @maxep, this is not making Kronos thread safe, is it? Wouldn't be better to add locks around stableTime?

[maxep]

I forgot about that one!

Indeed, it's not making stableTime thread-safe. It was more about allowing reading Clock.now on a different queue, to avoid syncing on main.
Making stableTime atomic would be best 👍