**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.
const DataPartPtr & part, Coordination::Requests & ops, String part_name, NameSet * absent_replicas_paths)
{
    if (part_name.empty())
        part_name = part->name;
    auto local_part_header = ReplicatedMergeTreePartHeader::fromColumnsAndChecksums(
        part->getColumns(), part->checksums);
    Strings replicas = zookeeper->getChildren(fs::path(zookeeper_path) / "replicas");
    std::shuffle(replicas.begin(), replicas.end(), thread_local_rng);
    bool has_been_already_added = false;
    for (const String & replica : replicas)
    {
        String current_part_path = fs::path(zookeeper_path) / "replicas" / replica / "parts" / part_name;
        String part_zk_str;
        if (!zookeeper->tryGet(current_part_path, part_zk_str))
        {
            if (absent_replicas_paths)
                absent_replicas_paths->emplace(current_part_path);
            continue;
        }
        ReplicatedMergeTreePartHeader replica_part_header;
        if (part_zk_str.empty())
        {
            String columns_str;
            String checksums_str;
            if (zookeeper->tryGet(fs::path(current_part_path) / "columns", columns_str) &&
                zookeeper->tryGet(fs::path(current_part_path) / "checksums", checksums_str))
            {
                replica_part_header = ReplicatedMergeTreePartHeader::fromColumnsAndChecksumsZNodes(columns_str, checksums_str);
            }
            else