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. 

{
        mark_cache_size = max_cache_size;
        LOG_INFO(log, "Mark cache size was lowered to {} because the system has low amount of memory",
            formatReadableSizeWithBinarySuffix(mark_cache_size));
    }
    global_context->setMarkCache(mark_cache_size, mark_cache_policy);
    
    size_t index_uncompressed_cache_size = config().getUInt64("index_uncompressed_cache_size", 0);
    if (index_uncompressed_cache_size)
        global_context->setIndexUncompressedCache(index_uncompressed_cache_size);
    
    size_t index_mark_cache_size = config().getUInt64("index_mark_cache_size", 0);
    if (index_mark_cache_size)
        global_context->setIndexMarkCache(index_mark_cache_size);
    
    size_t mmap_cache_size = config().getUInt64("mmap_cache_size", 1000);   
    if (mmap_cache_size)
        global_context->setMMappedFileCache(mmap_cache_size);
#if USE_EMBEDDED_COMPILER
    
    constexpr size_t compiled_expression_cache_size_default = 1024 * 1024 * 128;
    size_t compiled_expression_cache_size = config().getUInt64("compiled_expression_cache_size", compiled_expression_cache_size_default);
    constexpr size_t compiled_expression_cache_elements_size_default = 10000;
    size_t compiled_expression_cache_elements_size = config().getUInt64("compiled_expression_cache_elements_size", compiled_expression_cache_elements_size_default);
    CompiledExpressionCacheFactory::instance().init(compiled_expression_cache_size, compiled_expression_cache_elements_size);
#endif
    
    fs::path format_schema_path(config().getString("format_schema_path", path / "format_schemas/"));
    global_context->setFormatSchemaPath(format_schema_path);
    fs::create_directories(format_schema_path);