**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.
{
    return MinimalisticDataPartChecksums::getSerializedString(checksums,
        getSettings()->use_minimalistic_checksums_in_zookeeper);
}
MergeTreeData::MutableDataPartPtr StorageReplicatedMergeTree::attachPartHelperFoundValidPart(const LogEntry& entry) const
{
    const MergeTreePartInfo actual_part_info = MergeTreePartInfo::fromPartName(entry.new_part_name, format_version);
    const String part_new_name = actual_part_info.getPartName();
    for (const DiskPtr & disk : getStoragePolicy()->getDisks())
        for (const auto it = disk->iterateDirectory(fs::path(relative_data_path) / "detached/"); it->isValid(); it->next())
        {