**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 value;
        if (!zookeeper->tryGet(fs::path(zookeeper_path) / "replicas" / replica / "min_unprocessed_insert_time", value))
            continue;
        time_t replica_time = value.empty() ? 0 : parse<time_t>(value);
        if (replica_time == 0)
        {
            /** Note
              * The conclusion that the replica does not lag may be incorrect,
              *  because the information about `min_unprocessed_insert_time` is taken
              *  only from that part of the log that has been moved to the queue.
              * If the replica for some reason has stalled `queueUpdatingTask`,
              *  then `min_unprocessed_insert_time` will be incorrect.
              */
            have_replica_with_nothing_unprocessed = true;
            break;
        }
        if (replica_time > max_replicas_unprocessed_insert_time)
            max_replicas_unprocessed_insert_time = replica_time;
    }
    if (have_replica_with_nothing_unprocessed)
        out_relative_delay = out_absolute_delay;
    else
    {
        max_replicas_unprocessed_insert_time = std::min(current_time, max_replicas_unprocessed_insert_time);
        time_t min_replicas_delay = current_time - max_replicas_unprocessed_insert_time;
        if (out_absolute_delay > min_replicas_delay)
            out_relative_delay = out_absolute_delay - min_replicas_delay;
    }
}
void StorageReplicatedMergeTree::fetchPartition(