Implement a periodic task scheduler, and a basic dormant mode

This is a revised version of !397 (closed); it implements a scheduling system for periodic tasks that can be externally controlled, and then uses the external control aspect to implement a basic dormant mode (#90).

More technically, the scheduling system consists of a Stream that periodic tasks are expected to embed in a while loop or similar, a way for tasks themselves to choose how long to wait until the stream next yields a result, and a handle to control this outside of the task.