Prioritize blocked task messaging over idle tasks (#627)

* Prioritize blocked task messaging over idle tasks

* Add test_scheduler_schedule_execution_blocked

* Add commentary

* Reword commentary a bit

* Document not-chosen approach in detail

* Use {crossbeam,chained}_channel::unbounded()

* Add test for race condition around SchedulingContext

* Fix race condition around SchedulingContext

* Comment about finished channels

* Clean up the scheduling context race test

* Codify hidden assumption with extra 1 recv/session
  (EDIT there's actually no extra recv)