**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.
try
        {
            if (zookeeper_name == default_zookeeper_name)
            {
                current_zookeeper = getContext()->getZooKeeper();
            }
            else
            {
                current_zookeeper = getContext()->getAuxiliaryZooKeeper(zookeeper_name);
            }
        }
        catch (...)
        {
            if (!attach)
                dropIfEmpty();
            throw;
        }
    }
    bool skip_sanity_checks = false;
    
    if (attach)
    {
        if (current_zookeeper && current_zookeeper->exists(replica_path + "/flags/force_restore_data"))
        {