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. 

global_context->addWarningMessage("Server was built in debug mode. It will work slowly.");
#endif
    if (ThreadFuzzer::instance().isEffective())
        global_context->addWarningMessage("ThreadFuzzer is enabled. Application will run slowly and unstable.");
#if defined(SANITIZER)
    global_context->addWarningMessage("Server was built with sanitizer. It will work slowly.");
#endif
    sanityChecks(*this);
    
    
    
    GlobalThreadPool::initialize(
        config().getUInt("max_thread_pool_size", 10000),
        config().getUInt("max_thread_pool_free_size", 1000),
        config().getUInt("thread_pool_queue_size", 10000)
    );
    IOThreadPool::initialize(
        config().getUInt("max_io_thread_pool_size", 100),
        config().getUInt("max_io_thread_pool_free_size", 0),
        config().getUInt("io_thread_pool_queue_size", 10000));
    
    if (config().has("local_cache_for_remote_fs"))
    {
        bool enable = config().getBool("local_cache_for_remote_fs.enable", false);
        if (enable)
        {
            String root_dir = config().getString("local_cache_for_remote_fs.root_dir");
            UInt64 limit_size = config().getUInt64("local_cache_for_remote_fs.limit_size");
            UInt64 bytes_read_before_flush
                = config().getUInt64("local_cache_for_remote_fs.bytes_read_before_flush", DBMS_DEFAULT_BUFFER_SIZE);