**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.
{
    return has_lightweight_delete_parts.load(std::memory_order_relaxed);
}
void StorageReplicatedMergeTree::clearOldPartsAndRemoveFromZK()
{
    auto table_lock = lockForShare(
            RWLockImpl::NO_QUERY, getSettings()->lock_acquire_timeout_for_background_operations);
    auto zookeeper = getZooKeeper();
    
    
    DataPartsVector parts = grabOldParts();
    if (parts.empty())
        return;
    DataPartsVector parts_to_delete_only_from_filesystem;    
    DataPartsVector parts_to_delete_completely;              
    DataPartsVector parts_to_retry_deletion;                 
    DataPartsVector parts_to_remove_from_filesystem;         
    for (const auto & part : parts)
    {
        if (!part->is_duplicate)
            parts_to_delete_completely.emplace_back(part);
        else
            parts_to_delete_only_from_filesystem.emplace_back(part);