**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 (!covering_drop_range.empty())
        {
            LOG_TRACE(log, "{} {}: it's covered by DROP_RANGE {}", log_msg_context, part_name, covering_drop_range);
            return true;
        }
        
        
        
        String covering_get_part_entry = get_part_set.getContainingPart(part_name);
        if (covering_get_part_entry.empty())
            return false;
        if (covering_get_part_entry != part_name)
        {
            LOG_TRACE(log, "{} {}: it's covered by GET_PART {}", log_msg_context, part_name, covering_get_part_entry);
            return true;
        }
        
        
        
        
        
        if (created_gets.contains(part_name))
        {
            
            
            
            LOG_TRACE(log, "{} {}: GET_PART for it is already created", log_msg_context, part_name);
            return true;
        }
        return false;