Code review comment for lp:~andreas-pokorny/mir/fake-event-hub-to-fake-input-server-configuration

Especially given that start/stop isn't on a critical path, guarding against interleaving start/stop calls seems like something thats easy to do with a mutex... but if that's not the goal, it seems okay to guard against many stop() or start() calls this way. So I guess still needs fixing on the exception safety.