fix mock NMR deregister race.

[shankarseal]

Summary

• fix the mock NMR deregister race where a Start binding could be treated as already unbinding
• when a binding transitions to Ready/Ready, immediately start unbind if either side is already deregistering
• add a concurrent register/deregister smoke test that loops provider and client registration/deregistration and prints executed iteration counts

Testing

• run usersim_tests with filter: "concurrent register/deregister smoke"

[mikeagun]

Should we have some sort of synchronization to ensure the provider and client actually overlap, or at least counter(s) so we can review how much potential contention there was?

My concern is that in CI one thread could complete its iterations before the second thread starts.