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?
Session::startupNamedSessions();
        {
            String level_str = config().getString("text_log.level", "");
            int level = level_str.empty() ? INT_MAX : Poco::Logger::parseLevel(level_str);
            setTextLog(global_context->getTextLog(), level);
        }
        buildLoggers(config(), logger());
        main_config_reloader->start();
        access_control.startPeriodicReloadingUsersConfigs();
        if (dns_cache_updater)
            dns_cache_updater->start();
        {
            LOG_INFO(log, "Available RAM: {}; physical cores: {}; logical cores: {}.",
                formatReadableSizeWithBinarySuffix(memory_amount),
                getNumberOfPhysicalCPUCores(),  
                std::thread::hardware_concurrency());
        }
        
        try
        {
            global_context->loadDictionaries(config());
        }
        catch (...)
        {
            LOG_ERROR(log, "Caught exception while loading dictionaries.");
            throw;
        }
        if (has_zookeeper && config().has("distributed_ddl"))
        {