**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 = storage.getZooKeeper();
    String quorum_str;
    if (zookeeper->tryGet(fs::path(storage.zookeeper_path) / "quorum" / "status", quorum_str))
    {
        ReplicatedMergeTreeQuorumEntry quorum_entry(quorum_str);
        if (!quorum_entry.replicas.contains(storage.replica_name)
            && storage.getActiveContainingPart(quorum_entry.part_name))
        {
            LOG_WARNING(log, "We have part {} but we is not in quorum. Updating quorum. This shouldn't happen often.", quorum_entry.part_name);
            storage.updateQuorum(quorum_entry.part_name, false);
        }
    }
    Strings part_names;
    String parallel_quorum_parts_path = fs::path(storage.zookeeper_path) / "quorum" / "parallel";
    if (zookeeper->tryGetChildren(parallel_quorum_parts_path, part_names) == Coordination::Error::ZOK)
    {
        for (auto & part_name : part_names)
        {
            if (zookeeper->tryGet(fs::path(parallel_quorum_parts_path) / part_name, quorum_str))
            {
                ReplicatedMergeTreeQuorumEntry quorum_entry(quorum_str);
                if (!quorum_entry.replicas.contains(storage.replica_name)
                    && storage.getActiveContainingPart(part_name))
                {
                    LOG_WARNING(log, "We have part {} but we is not in quorum. Updating quorum. This shouldn't happen often.", part_name);
                    storage.updateQuorum(part_name, true);
                }
            }
        }