**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();
    return zookeeper->exists(fs::path(zookeeper_path) / "replicas" / replica / "parts" / part_name);
}
String StorageReplicatedMergeTree::findReplicaHavingPart(const String & part_name, bool active)
{
    auto zookeeper = getZooKeeper();
    Strings replicas = zookeeper->getChildren(fs::path(zookeeper_path) / "replicas");
    
    std::shuffle(replicas.begin(), replicas.end(), thread_local_rng);
    LOG_TRACE(log, "Candidate replicas: {}", replicas.size());
    for (const String & replica : replicas)
    {
        
        if (replica == replica_name)
            continue;
        LOG_TRACE(log, "Candidate replica: {}", replica);
        if (checkReplicaHavePart(replica, part_name) &&
            (!active || zookeeper->exists(fs::path(zookeeper_path) / "replicas" / replica / "is_active")))
            return replica;
        
    }
    return {};
}
String StorageReplicatedMergeTree::findReplicaHavingCoveringPart(LogEntry & entry, bool active)