**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.
part = get_part();
        if (part->data_part_storage->getDiskName() != replaced_disk->getName())
            throw Exception("Part " + part->name + " fetched on wrong disk " + part->data_part_storage->getDiskName(), ErrorCodes::LOGICAL_ERROR);
        auto replaced_path = fs::path(replaced_part_path);
        auto builder = part->data_part_storage->getBuilder();
        builder->rename(replaced_path.parent_path(), replaced_path.filename(), nullptr, true, false);
        part->data_part_storage->onRename(replaced_path.parent_path(), replaced_path.filename());
        builder->commit();
    }
    catch (const Exception & e)
    {
        
        
        if (e.code() == ErrorCodes::DIRECTORY_ALREADY_EXISTS)
        {
            LOG_TRACE(log, "Not fetching part: {}", e.message());
            return nullptr;
        }
        throw;
    }
    catch (...)
    {
        write_part_log(ExecutionStatus::fromCurrentException());
        throw;
    }
    ProfileEvents::increment(ProfileEvents::ReplicatedPartFetches);
    LOG_DEBUG(log, "Fetched part {} from {}", part_name, source_replica_path);
    return part->data_part_storage;
}
void StorageReplicatedMergeTree::startup()