Schedulers shutdown capability.

Proposal for shutting down the computation and io schedulers on demand.

#1730
I don't like reusing the `Subscription` idea for this as it confuses with the lifecycle that we use `Subscription` for everywhere else. 

This is very much an internal mechanism for shutting down or resetting, not a normal lifecycle event. No one "subscribes" to a thread pool, so unsubscribing to shut down the pool is confusing.