**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.
|| (log_pointer == max_log_pointer && queue_size < min_queue_size))
            {
                max_log_pointer = log_pointer;
                min_queue_size = queue_size;
                best_replica = replica;
            }
        }
    }
    if (best_replica.empty())
        throw Exception("Logical error: cannot choose best replica.", ErrorCodes::LOGICAL_ERROR);
    LOG_INFO(log, "Found {} replicas, {} of them are active. Selected {} to fetch from.", replicas.size(), active_replicas.size(), best_replica);
    String best_replica_path = fs::path(from) / "replicas" / best_replica;
    
    /** Trying to download these parts.
      * Some of them could be deleted due to the merge.
      * In this case, update the information about the available parts and try again.
      */
    unsigned try_no = 0;
    Strings missing_parts;
    do
    {
        if (try_no)
            LOG_INFO(log, "Some of parts ({}) are missing. Will try to fetch covering parts.", missing_parts.size());
        if (try_no >= query_context->getSettings().max_fetch_partition_retries_count)
            throw Exception("Too many retries to fetch parts from " + best_replica_path, ErrorCodes::TOO_MANY_RETRIES_TO_FETCH_PARTS);
        Strings parts = zookeeper->getChildren(fs::path(best_replica_path) / "parts");
        ActiveDataPartSet active_parts_set(format_version, parts);
        Strings parts_to_fetch;
        if (missing_parts.empty())
        {