Ordinary database engine is deprecated but there is no simple mean to change database engine to Atomic for existing databases.

Possible options:
- ALTER DATABASE MODIFY ENGINE query.
- Flag that triggers migration at ClickHouse startup.

The latter should be easier to implement. 

ConcurrencyControl::instance().setMaxConcurrency(ConcurrencyControl::Unlimited);
            }
            else
                ConcurrencyControl::instance().setMaxConcurrency(ConcurrencyControl::Unlimited);
            if (config->has("max_concurrent_queries"))
                global_context->getProcessList().setMaxSize(config->getInt("max_concurrent_queries", 0));
            if (config->has("max_concurrent_insert_queries"))
                global_context->getProcessList().setMaxInsertQueriesAmount(config->getInt("max_concurrent_insert_queries", 0));
            if (config->has("max_concurrent_select_queries"))
                global_context->getProcessList().setMaxSelectQueriesAmount(config->getInt("max_concurrent_select_queries", 0));
            if (config->has("keeper_server"))
                global_context->updateKeeperConfiguration(*config);
            
            
            
            
            if (global_context->areBackgroundExecutorsInitialized() && (config->has("background_pool_size") || config->has("background_merges_mutations_concurrency_ratio")))
            {
                auto new_pool_size = config->getUInt64("background_pool_size", 16);
                auto new_ratio = config->getUInt64("background_merges_mutations_concurrency_ratio", 2);
                global_context->getMergeMutateExecutor()->increaseThreadsAndMaxTasksCount(new_pool_size, new_pool_size * new_ratio);
            }
            if (global_context->areBackgroundExecutorsInitialized() && config->has("background_move_pool_size"))
            {
                auto new_pool_size = config->getUInt64("background_move_pool_size");
                global_context->getMovesExecutor()->increaseThreadsAndMaxTasksCount(new_pool_size, new_pool_size);
            }
            if (global_context->areBackgroundExecutorsInitialized() && config->has("background_fetches_pool_size"))
            {
                auto new_pool_size = config->getUInt64("background_fetches_pool_size");