**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.
zkutil::ZooKeeperPtr & zookeeper, const Strings & part_names, NameSet * parts_should_be_retried)
{
    std::vector<std::future<Coordination::ExistsResponse>> exists_futures;
    std::vector<std::future<Coordination::MultiResponse>> remove_futures;
    exists_futures.reserve(part_names.size());
    remove_futures.reserve(part_names.size());
    try
    {
        
        
        for (const String & part_name : part_names)
        {
            String part_path = fs::path(replica_path) / "parts" / part_name;
            exists_futures.emplace_back(zookeeper->asyncExists(part_path));
        }
        for (size_t i = 0; i < part_names.size(); ++i)
        {
            Coordination::ExistsResponse exists_resp = exists_futures[i].get();
            if (exists_resp.error == Coordination::Error::ZOK)
            {
                Coordination::Requests ops;
                removePartFromZooKeeper(part_names[i], ops, exists_resp.stat.numChildren > 0);
                remove_futures.emplace_back(zookeeper->asyncTryMultiNoThrow(ops));
            }
            else
            {
                LOG_DEBUG(log, "There is no part {} in ZooKeeper, it was only in filesystem", part_names[i]);
                
                remove_futures.emplace_back();
            }