Write a file `uuid` in server's data directory that contains random UUID, generated at startup if file does not exist.

It will be used for several purposes:
- to anonymously identify server in Sentry (currently Sentry is unusable);
- (possible future usage) to identify server in ensemble.

@tavplubix comments?
LOG_INFO(log, "Closed connections. But {} remain."
                    " Tip: To increase wait time add to config: <shutdown_wait_unfinished>60</shutdown_wait_unfinished>", current_connections);
            else
                LOG_INFO(log, "Closed connections.");
            dns_cache_updater.reset();
            main_config_reloader.reset();
            if (current_connections)
            {
                
                
                
                
                
                dumpCoverageReportIfPossible();
                LOG_INFO(log, "Will shutdown forcefully.");
                forceShutdown();
            }
        });
        std::vector<std::unique_ptr<MetricsTransmitter>> metrics_transmitters;
        for (const auto & graphite_key : DB::getMultipleKeysFromConfig(config(), "", "graphite"))
        {
            metrics_transmitters.emplace_back(std::make_unique<MetricsTransmitter>(
                global_context->getConfigRef(), graphite_key, async_metrics));
        }
        waitForTerminationRequest();
    }
    return Application::EXIT_OK;
}
}