**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.
|| MergeTreePartInfo::contains(part_on_replica, largest_part_found, format_version))
                {
                    largest_part_found = part_on_replica;
                }
            }
        }
        if (!largest_part_found.empty())
        {
            bool the_same_part = largest_part_found == entry.new_part_name;
            
            if (!the_same_part)
            {
                String reject_reason;
                if (!queue.addFuturePartIfNotCoveredByThem(largest_part_found, entry, reject_reason))
                {
                    LOG_INFO(log, "Will not fetch part {} covering {}. {}", largest_part_found, entry.new_part_name, reject_reason);
                    return {};
                }
            }
            return replica;
        }
    }
    return {};
}
String StorageReplicatedMergeTree::findReplicaHavingCoveringPart(
    const String & part_name, bool active, String & found_part_name)
{
    auto zookeeper = getZooKeeper();
    Strings replicas = zookeeper->getChildren(fs::path(zookeeper_path) / "replicas");