**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.
{
    
    if (!has_metadata_in_zookeeper.has_value() || !*has_metadata_in_zookeeper)
        return;
    try
    {
        InterserverIOEndpointPtr data_parts_exchange_ptr = std::make_shared<DataPartsExchange::Service>(*this);
        [[maybe_unused]] auto prev_ptr = std::atomic_exchange(&data_parts_exchange_endpoint, data_parts_exchange_ptr);
        assert(prev_ptr == nullptr);
        getContext()->getInterserverIOHandler().addEndpoint(data_parts_exchange_ptr->getId(replica_path), data_parts_exchange_ptr);
        startBeingLeader();
        
        restarting_thread.start();
        
        
        
        
        startup_event.wait();
        startBackgroundMovesIfNeeded();
        part_moves_between_shards_orchestrator.start();
    }
    catch (...)
    {
        
        
        
        try