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. 

auto * global_overcommit_tracker = global_context->getGlobalOvercommitTracker();
            total_memory_tracker.setOvercommitTracker(global_overcommit_tracker);
            
            
            
            updateLevels(*config, logger());
            global_context->setClustersConfig(config, has_zookeeper);
            global_context->setMacros(std::make_unique<Macros>(*config, "macros", log));
            global_context->setExternalAuthenticatorsConfig(*config);
            global_context->loadOrReloadDictionaries(*config);
            global_context->loadOrReloadModels(*config);
            global_context->loadOrReloadUserDefinedExecutableFunctions(*config);
            global_context->setRemoteHostFilter(*config);
            
            if (config->has("max_table_size_to_drop"))
                global_context->setMaxTableSizeToDrop(config->getUInt64("max_table_size_to_drop"));
            if (config->has("max_partition_size_to_drop"))
                global_context->setMaxPartitionSizeToDrop(config->getUInt64("max_partition_size_to_drop"));
            if (config->has("concurrent_threads_soft_limit"))
            {
                auto concurrent_threads_soft_limit = config->getInt("concurrent_threads_soft_limit", 0);
                if (concurrent_threads_soft_limit == -1)
                {
                    
                    constexpr size_t thread_factor = 3;
                    concurrent_threads_soft_limit = std::thread::hardware_concurrency() * thread_factor;
                }
                if (concurrent_threads_soft_limit)
                    ConcurrencyControl::instance().setMaxConcurrency(concurrent_threads_soft_limit);
                else