**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.
currently_fetching_parts.erase(part_name);
    });
    LOG_DEBUG(log, "Fetching part {} from {}", part_name, source_replica_path);
    TableLockHolder table_lock_holder;
    if (!to_detached)
        table_lock_holder = lockForShare(RWLockImpl::NO_QUERY, getSettings()->lock_acquire_timeout_for_background_operations);
    
    Stopwatch stopwatch;
    MutableDataPartPtr part;
    DataPartsVector replaced_parts;
    auto write_part_log = [&] (const ExecutionStatus & execution_status)
    {
        writePartLog(
            PartLogElement::DOWNLOAD_PART, execution_status, stopwatch.elapsed(),
            part_name, part, replaced_parts, nullptr);
    };
    DataPartPtr part_to_clone;
    {
        
        
        
        auto covered_part_info = part_info;
        covered_part_info.mutation = 0;
        auto source_part = getActiveContainingPart(covered_part_info);
        if (source_part)
        {
            auto source_part_header = ReplicatedMergeTreePartHeader::fromColumnsAndChecksums(
                source_part->getColumns(), source_part->checksums);
            String part_path = fs::path(source_replica_path) / "parts" / part_name;
            String part_znode = zookeeper->get(part_path);