**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.
{
    String source_metadata_version_str;
    bool metadata_version_exists = zookeeper->tryGet(source_path + "/metadata_version", source_metadata_version_str);
    if (!metadata_version_exists)
    {
        
        
        LOG_WARNING(log, "Node {} does not exist. "
                         "Most likely it's because too old version of ClickHouse is running on replica {}. "
                         "Will not check metadata consistency",
                         source_path + "/metadata_version", source_replica);
        return;
    }
    Int32 source_metadata_version = parse<Int32>(source_metadata_version_str);
    if (metadata_version == source_metadata_version)
        return;