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"