**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.
auto zookeeper = getZooKeeper();
        Transaction transaction(*this, NO_TRANSACTION_RAW);
        Coordination::Requests ops;
        for (PartDescriptionPtr & part_desc : final_parts)
        {
            auto builder = part_desc->res_part->data_part_storage->getBuilder();
            renameTempPartAndReplace(part_desc->res_part, transaction, builder);
            getCommitPartOps(ops, part_desc->res_part);
            lockSharedData(*part_desc->res_part, false, part_desc->hardlinked_files);
        }
        if (!ops.empty())
            zookeeper->multi(ops);
        {
            auto data_parts_lock = lockParts();
            transaction.commit(&data_parts_lock);
            if (replace)
            {
                parts_to_remove = removePartsInRangeFromWorkingSetAndGetPartsToRemoveFromZooKeeper(NO_TRANSACTION_RAW, drop_range, data_parts_lock);
                String parts_to_remove_str;
                for (const auto & part : parts_to_remove)
                {
                    parts_to_remove_str += part->name;
                    parts_to_remove_str += " ";
                }
                LOG_TRACE(log, "Replacing {} parts {}with {} parts {}", parts_to_remove.size(), parts_to_remove_str,
                          final_parts.size(), boost::algorithm::join(final_part_names, ", "));
            }
        }
        PartLog::addNewParts(getContext(), res_parts, watch.elapsed());
    }