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. 

database_catalog.assertDatabaseExists(default_database);
    }
    catch (...)
    {
        tryLogCurrentException(log, "Caught exception while loading metadata");
        throw;
    }
    LOG_DEBUG(log, "Loaded metadata.");
    
    
#if USE_UNWIND && !WITH_COVERAGE
    
    if (hasPHDRCache())
    {
        global_context->initializeTraceCollector();
        
        UInt64 total_memory_profiler_step = config().getUInt64("total_memory_profiler_step", 0);
        if (total_memory_profiler_step)
        {
            total_memory_tracker.setProfilerStep(total_memory_profiler_step);
        }
        double total_memory_tracker_sample_probability = config().getDouble("total_memory_tracker_sample_probability", 0);
        if (total_memory_tracker_sample_probability)
        {
            total_memory_tracker.setSampleProbability(total_memory_tracker_sample_probability);
        }
    }
#endif
    
#if !USE_UNWIND