**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.
if (children.empty())
        {
            
            error_code = zookeeper_ptr->tryRemove(zookeeper_part_node);
            if (error_code == Coordination::Error::ZOK)
            {
                LOG_TRACE(logger, "Removed last parent zookeeper lock {} for part {} (part is finally unlocked)", zookeeper_part_uniq_node, part_name);
            }
            else if (error_code == Coordination::Error::ZNOTEMPTY)
            {
                LOG_TRACE(logger, "Cannot remove last parent zookeeper lock {} for part {}, another replica locked part concurrently", zookeeper_part_uniq_node, part_name);
            }
            else if (error_code == Coordination::Error::ZNONODE)
            {
                LOG_TRACE(logger, "Node with parent zookeeper lock {} for part {} doesn't exist (part was unlocked before)", zookeeper_part_uniq_node, part_name);
            }
            else
            {
                throw zkutil::KeeperException(error_code, zookeeper_part_uniq_node);
            }
        }
        else
        {
            LOG_TRACE(logger, "Can't remove parent zookeeper lock {} for part {}, because children {} ({}) were concurrently created",
                      zookeeper_part_node, part_name, children.size(), fmt::join(children, ", "));
        }
    }
    return std::make_pair(part_has_no_more_locks, files_not_to_remove);
}
DataPartStoragePtr StorageReplicatedMergeTree::tryToFetchIfShared(