Store ~Task in thread local storage instead of ~Scheduler

The current design places a ~Scheduler in TLS. This turned out to be a bad idea because now I want to have tasks that don't use a scheduler. Instead we should place ~Task in TLS. Once we do that we'll need a place to store the ~Scheduler instance when running in scheduler context, so we'll have to use a ~Task, making the scheduler itself a non-green thread task.
