**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.
Strings replicas = zookeeper->getChildren(fs::path(zookeeper_path) / "replicas");
    std::vector<zkutil::ZooKeeper::FutureGet> futures;
    for (const String & source_replica_name : replicas)
    {
        
        if (source_replica_name == replica_name)
            continue;
        String source_replica_path = fs::path(zookeeper_path) / "replicas" / source_replica_name;
        
        
        futures.emplace_back(zookeeper->asyncTryGet(fs::path(source_replica_path) / "is_lost"));
        futures.emplace_back(zookeeper->asyncTryGet(fs::path(source_replica_path) / "log_pointer"));
        futures.emplace_back(zookeeper->asyncTryGet(fs::path(source_replica_path) / "queue"));
    }
    
    for (auto & future : futures)
        future.wait();
    Strings log_entries = zookeeper->getChildren(fs::path(zookeeper_path) / "log");
    size_t max_log_entry = 0;
    if (!log_entries.empty())
    {
        String last_entry = *std::max_element(log_entries.begin(), log_entries.end());
        max_log_entry = parse<UInt64>(last_entry.substr(strlen("log-")));
    }
    
    ++max_log_entry;
    size_t min_replication_lag = std::numeric_limits<size_t>::max();
    String source_replica;