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. 

{
            LOG_DEBUG(log, "rlimit on number of threads is {}", rlim.rlim_cur);
        }
        else
        {
            rlim_t old = rlim.rlim_cur;
            rlim.rlim_cur = rlim.rlim_max;
            int rc = setrlimit(RLIMIT_NPROC, &rlim);
            if (rc != 0)
            {
                LOG_WARNING(log, "Cannot set max number of threads to {}. error: {}", rlim.rlim_cur, strerror(errno));
                rlim.rlim_cur = old;
            }
            else
            {
                LOG_DEBUG(log, "Set max number of threads to {} (was {}).", rlim.rlim_cur, old);
            }
        }
        if (rlim.rlim_cur < 30000)
        {
            global_context->addWarningMessage("Maximum number of threads is lower than 30000. There could be problems with handling a lot of simultaneous queries.");
        }
    }
    static ServerErrorHandler error_handler;
    Poco::ErrorHandler::set(&error_handler);
    
    LOG_DEBUG(log, "Initializing DateLUT.");
    DateLUT::instance();
    LOG_TRACE(log, "Initialized DateLUT with time zone '{}'.", DateLUT::instance().getTimeZone());