thread state coordinator: added simpler deferred stop notification method.

Now that ThreadStateCoordinator errors out on threads in unexpected states,
it has enough information to know which threads need stop requests fired
when we want to do a deferred callback on a thread's behalf.  This change
adds a new method, CallAfterRunningThreadsStop(...), which no longer
takes a set of thread ids that require stop requests.  It's much harder
to misuse this method and (with newer error logic) it's harder to
correctly use the original method.  Expect the original method that takes
the set of thread ids to stop to disappear in the near future.

Adds several tests for CallAfterRunningThreadsStop().