Refactor Stoppable:
The Stoppable interface aids in the enforcement of invariants needed to
successful start and stop a multi-threaded application composed of classes
that depend on each other in complex ways.
* Test written to confirm the current behavior.
* Comments updated to reflect the current behavior.
* Public API reduced to what is currently in use.
* Protected data members made private.
* volatile bool members changed to std::atomic<bool>.
* std::atomic<int> members changed to std::atomic<bool>.
* Name storage uses std::string