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?
loadMetadataSystem(global_context);
        
        global_context->initializeSystemLogs();
        global_context->setSystemZooKeeperLogAfterInitializationIfNeeded();
        auto & database_catalog = DatabaseCatalog::instance();
        
        attachSystemTablesServer(*database_catalog.getSystemDatabase(), has_zookeeper);
        
        database_catalog.initializeAndLoadTemporaryDatabase();
        
        loadMetadata(global_context, default_database);
        database_catalog.loadDatabases();
        
        database_catalog.assertDatabaseExists(default_database);
    }
    catch (...)
    {
        tryLogCurrentException(log, "Caught exception while loading metadata");
        throw;
    }
    LOG_DEBUG(log, "Loaded metadata.");