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. 

assignee.scheduleFetchTask(std::make_shared<ExecutableLambdaAdapter>(
            [this, selected_entry] () mutable
            {
                return processQueueEntry(selected_entry);
            }, common_assignee_trigger, getStorageID()));
        return true;
    }
    else if (job_type == LogEntry::MERGE_PARTS)
    {
        auto task = std::make_shared<MergeFromLogEntryTask>(selected_entry, *this, common_assignee_trigger);
        assignee.scheduleMergeMutateTask(task);
        return true;
    }
    else if (job_type == LogEntry::MUTATE_PART)
    {
        auto task = std::make_shared<MutateFromLogEntryTask>(selected_entry, *this, common_assignee_trigger);
        assignee.scheduleMergeMutateTask(task);
        return true;
    }
    else
    {
        assignee.scheduleCommonTask(std::make_shared<ExecutableLambdaAdapter>(
            [this, selected_entry] () mutable
            {
                return processQueueEntry(selected_entry);
            }, common_assignee_trigger, getStorageID()),  true);
        return true;
    }
}
bool StorageReplicatedMergeTree::canExecuteFetch(const ReplicatedMergeTreeLogEntry & entry, String & disable_reason) const