**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();
    
    String quorum_status_path = fs::path(zookeeper_path) / "quorum" / "status";
    if (is_parallel)
        quorum_status_path = fs::path(zookeeper_path) / "quorum" / "parallel" / part_name;
    
    const String quorum_last_part_path = fs::path(zookeeper_path) / "quorum" / "last_part";
    String value;
    Coordination::Stat stat;
    
    while (zookeeper->tryGet(quorum_status_path, value, &stat))
    {
        ReplicatedMergeTreeQuorumEntry quorum_entry(value);
        if (quorum_entry.part_name != part_name)
        {
            LOG_TRACE(log, "Quorum {}, already achieved for part {} current part {}",
                      quorum_status_path, part_name, quorum_entry.part_name);
            
            break;
        }
        quorum_entry.replicas.insert(replica_name);
        if (quorum_entry.replicas.size() >= quorum_entry.required_number_of_replicas)
        {
            
            LOG_TRACE(log, "Got {} replicas confirmed quorum {}, going to remove node",
                      quorum_entry.replicas.size(), quorum_status_path);
            Coordination::Requests ops;
            Coordination::Responses responses;