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->getFetchesExecutor()->increaseThreadsAndMaxTasksCount(new_pool_size, new_pool_size);
            }
            if (global_context->areBackgroundExecutorsInitialized() && config->has("background_common_pool_size"))
            {
                auto new_pool_size = config->getUInt64("background_common_pool_size");
                global_context->getCommonExecutor()->increaseThreadsAndMaxTasksCount(new_pool_size, new_pool_size);
            }
            if (config->has("background_buffer_flush_schedule_pool_size"))
            {
                auto new_pool_size = config->getUInt64("background_buffer_flush_schedule_pool_size");
                global_context->getBufferFlushSchedulePool().increaseThreadsCount(new_pool_size);
            }
            if (config->has("background_schedule_pool_size"))
            {
                auto new_pool_size = config->getUInt64("background_schedule_pool_size");
                global_context->getSchedulePool().increaseThreadsCount(new_pool_size);
            }
            if (config->has("background_message_broker_schedule_pool_size"))
            {
                auto new_pool_size = config->getUInt64("background_message_broker_schedule_pool_size");
                global_context->getMessageBrokerSchedulePool().increaseThreadsCount(new_pool_size);
            }
            if (config->has("background_distributed_schedule_pool_size"))
            {
                auto new_pool_size = config->getUInt64("background_distributed_schedule_pool_size");
                global_context->getDistributedSchedulePool().increaseThreadsCount(new_pool_size);
            }
            if (!initial_loading)
            {