thread: remove harmless race on status_ in End()

if a thread was still doing work when End() was called there'd be a race
on worker->status_. in these cases, however, the specific value is
meaningless as it would be >= OK and the thread would have been shut
down properly, but we'll check 'impl_' instead to avoid any potential
TSan/DRD reports.

Change-Id: Ib93cbc226a099f07761f7bad765549dffb8054b1