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. 

{
    return has_lightweight_delete_parts.load(std::memory_order_relaxed);
}
void StorageReplicatedMergeTree::clearOldPartsAndRemoveFromZK()
{
    auto table_lock = lockForShare(
            RWLockImpl::NO_QUERY, getSettings()->lock_acquire_timeout_for_background_operations);
    auto zookeeper = getZooKeeper();
    
    
    DataPartsVector parts = grabOldParts();
    if (parts.empty())
        return;
    DataPartsVector parts_to_delete_only_from_filesystem;    
    DataPartsVector parts_to_delete_completely;              
    DataPartsVector parts_to_retry_deletion;                 
    DataPartsVector parts_to_remove_from_filesystem;         
    for (const auto & part : parts)
    {
        if (!part->is_duplicate)
            parts_to_delete_completely.emplace_back(part);
        else
            parts_to_delete_only_from_filesystem.emplace_back(part);