**Describe the solution you'd like**

We already can run clickhouse-server without configured ZooKeeper (the tables will start in read-only mode)
and with configured ZooKeeper, if it starts to be inaccessible at runtime (the tables will be read-only until the session is re-established).

Let's also make it possible to start up when ZooKeeper is unavailable.

**Additional context**

If that will be very hard to implement, we can consider other options:
- lazy attaching of the tables after sever start-up;
- a maintenance mode when the tables are not attached at start-up and require manual ATTACH queries.
}
    parts.clear();
    auto delete_parts_from_fs_and_rollback_in_case_of_error = [this] (const DataPartsVector & parts_to_delete, const String & parts_type)
    {
        NameSet parts_failed_to_delete;
        clearPartsFromFilesystem(parts_to_delete, false, &parts_failed_to_delete);
        DataPartsVector finally_remove_parts;
        if (!parts_failed_to_delete.empty())
        {
            DataPartsVector rollback_parts;
            for (const auto & part : parts_to_delete)
            {
                if (!parts_failed_to_delete.contains(part->name))
                    finally_remove_parts.push_back(part);
                else
                    rollback_parts.push_back(part);
            }
            if (!rollback_parts.empty())
                rollbackDeletingParts(rollback_parts);
        }
        else  
        {
            finally_remove_parts = parts_to_delete;
        }
        try
        {
            removePartsFinally(finally_remove_parts);
            LOG_DEBUG(log, "Removed {} {} parts", finally_remove_parts.size(), parts_type);
        }
        catch (...)