**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.
catch (...)
    {
        PartLog::addNewParts(getContext(), res_parts, watch.elapsed(), ExecutionStatus::fromCurrentException());
        for (const auto & res_part : res_parts)
            unlockSharedData(*res_part);
        throw;
    }
    removePartsFromZooKeeperWithRetries(parts_to_remove);
    res_parts.clear();
    parts_to_remove.clear();
    cleanup_thread.wakeup();
    return true;
}
void StorageReplicatedMergeTree::executeClonePartFromShard(const LogEntry & entry)
{
    auto zookeeper = getZooKeeper();
    Strings replicas = zookeeper->getChildren(entry.source_shard + "/replicas");
    std::shuffle(replicas.begin(), replicas.end(), thread_local_rng);
    String replica;
    for (const String & candidate : replicas)
    {
        if (zookeeper->exists(entry.source_shard + "/replicas/" + candidate + "/is_active"))
        {
            replica = candidate;
            break;
        }
    }
    if (replica.empty())
        throw Exception(ErrorCodes::NO_REPLICA_HAS_PART, "Not found active replica on shard {} to clone part {}", entry.source_shard, entry.new_part_name);
    LOG_INFO(log, "Will clone part from shard {} and replica {}", entry.source_shard, replica);