**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.
continue;
        }
        else
        {
            zkutil::KeeperMultiException::check(rc, ops, responses);
        }
    }
    ::sort(source_queue_names.begin(), source_queue_names.end());
    struct QueueEntryInfo
    {
        String data = {};
        Coordination::Stat stat = {};
        LogEntryPtr parsed_entry = {};
    };
    
    
    
    
    
    
    
    std::vector<QueueEntryInfo> source_queue;
    ActiveDataPartSet get_part_set{format_version};
    ActiveDataPartSet drop_range_set{format_version};
    {
        std::vector<zkutil::ZooKeeper::FutureGet> queue_get_futures;
        queue_get_futures.reserve(source_queue_names.size());
        for (const String & entry_name : source_queue_names)
            queue_get_futures.push_back(zookeeper->asyncTryGet(fs::path(source_path) / "queue" / entry_name));
        source_queue.reserve(source_queue_names.size());