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. 

ContextMutablePtr local_context, LoadingStrictnessLevel mode, bool skip_startup_tables)
{
    /** Tables load faster if they are loaded in sorted (by name) order.
      * Otherwise (for the ext4 filesystem), `DirectoryIterator` iterates through them in some order,
      *  which does not correspond to order tables creation and does not correspond to order of their location on disk.
      */
    ParsedTablesMetadata metadata;
    bool force_attach = LoadingStrictnessLevel::FORCE_ATTACH <= mode;
    loadTablesMetadata(local_context, metadata, force_attach);
    size_t total_tables = metadata.parsed_tables.size() - metadata.total_dictionaries;
    AtomicStopwatch watch;
    std::atomic<size_t> dictionaries_processed{0};
    std::atomic<size_t> tables_processed{0};
    ThreadPool pool;
    
    
    
    
    
    
    
    for (const auto & name_with_path_and_query : metadata.parsed_tables)
    {
        const auto & name = name_with_path_and_query.first;
        const auto & path = name_with_path_and_query.second.path;
        const auto & ast = name_with_path_and_query.second.ast;