**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 & part_name,
    const StorageMetadataPtr & metadata_snapshot,
    const String & source_replica_path,
    DiskPtr replaced_disk,
    String replaced_part_path)
{
    auto zookeeper = getZooKeeper();
    const auto part_info = MergeTreePartInfo::fromPartName(part_name, format_version);
    if (auto part = getPartIfExists(part_info, {MergeTreeDataPartState::Outdated, MergeTreeDataPartState::Deleting}))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 nullptr;
    }
    {
        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 nullptr;
        }
    }
    SCOPE_EXIT_MEMORY
    ({
        std::lock_guard lock(currently_fetching_parts_mutex);
        currently_fetching_parts.erase(part_name);
    });
    LOG_DEBUG(log, "Fetching part {} from {}", part_name, source_replica_path);
    TableLockHolder table_lock_holder = lockForShare(RWLockImpl::NO_QUERY, getSettings()->lock_acquire_timeout_for_background_operations);