Give one last tickle to terminated registries
Give one last tickle to terminated registries

`Registry::terminate` was merely setting `terminate_latch`, leaving
sleeping threads in an eternal slumber.  Normally that would just amount
to a resource leak, but `thread_pool::test::workers_stop` will stall
waiting for every thread to stop.  We just need a `tickle` to make sure
every thread will wake up and exit.

Fixes #200.

cuviper/rayon,cuviper/rayon