**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.
const String & source_replica_path, bool to_detached, size_t quorum, zkutil::ZooKeeper::Ptr zookeeper_, bool try_fetch_shared)
{
    auto zookeeper = zookeeper_ ? zookeeper_ : getZooKeeper();
    const auto part_info = MergeTreePartInfo::fromPartName(part_name, format_version);
    if (!to_detached)
    {
        if (auto part = getPartIfExists(part_info, {MergeTreeDataPartState::Outdated, MergeTreeDataPartState::Deleting}))
        {
            LOG_DEBUG(log, "Part {} should be deleted after previous attempt before fetch", part->name);
            
            cleanup_thread.wakeup();
            return false;
        }
    }
    {
        std::lock_guard lock(currently_fetching_parts_mutex);
        if (!currently_fetching_parts.insert(part_name).second)
        {
            LOG_DEBUG(log, "Part {} is already fetching right now", part_name);
            return false;
        }
    }
    SCOPE_EXIT_MEMORY
    ({
        std::lock_guard lock(currently_fetching_parts_mutex);