benchmarks: check shutdown after acquiring a token, and weak error messages on shutdown

[carl-mastrangelo]

This makes the shutdown status checked AFTER acquiring a token, which may take a long time. Tokens are released after a call finished, but acquired after checking the shutdown status. This makes the last calls of the benchmark run always fail loudly. Instead, check after getting the right of way and just give it back if the client is stopping.

Additionally, errors on shutdown aren't really important, and typically are just noise. Weaken them to FINE level if the driver is shutdown.

Updates #2372