**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();
    Coordination::Stat stat;
    String s = zookeeper->get(zookeeper_path + "/pinned_part_uuids", &stat);
    std::lock_guard lock(pinned_part_uuids_mutex);
    
    if (pinned_part_uuids->stat.version < stat.version)
    {
        auto new_pinned_part_uuids = std::make_shared<PinnedPartUUIDs>();
        new_pinned_part_uuids->fromString(s);
        new_pinned_part_uuids->stat = stat;
        pinned_part_uuids = new_pinned_part_uuids;
    }
}
void StorageReplicatedMergeTree::checkPartChecksumsAndAddCommitOps(const zkutil::ZooKeeperPtr & zookeeper,