mongos shutdown is non-deterministic when the shutdown command is executed

Once we signalShutdown() in exitCleanly, we're in a race to shutdown between the main thread (which will return from mongoSMain and call quickExit) and the thread handling the connection, which will join all the task executor threads and then call dbExit(). Depending on which thread wins, we get different logging output, since dbExit logs the process' exit code before returning. This is causing exit_logging.js to be flaky.