**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 (entry.alter_version < metadata_version)
    {
        
        
        
        LOG_WARNING(log, "Attempt to update metadata of version {} "
                         "to older version {} when processing log entry {}: {}",
                         metadata_version, entry.alter_version, entry.znode_name, entry.toString());
        return true;
    }
    auto zookeeper = getZooKeeper();
    auto columns_from_entry = ColumnsDescription::parse(entry.columns_str);
    auto metadata_from_entry = ReplicatedMergeTreeTableMetadata::parse(entry.metadata_str);
    MergeTreeData::DataParts parts;
    
    Coordination::Requests requests;
    requests.emplace_back(zkutil::makeSetRequest(fs::path(replica_path) / "columns", entry.columns_str, -1));
    requests.emplace_back(zkutil::makeSetRequest(fs::path(replica_path) / "metadata", entry.metadata_str, -1));
    auto table_id = getStorageID();
    auto alter_context = getContext();
    auto database = DatabaseCatalog::instance().getDatabase(table_id.database_name);
    bool is_in_replicated_database = database->getEngineName() == "Replicated";
    if (is_in_replicated_database)
    {
        auto mutable_alter_context = Context::createCopy(getContext());