**Describe the bug**
The ClickHouse client freezes when executing `ALTER TABLE` with `ADD COLUMN` and `MODIFY ORDER` on a table built using Replicated Versioned Collapsing Merge Tree engine.

**How to reproduce**
* ClickHouse client version 20.10.1.1.
Connecting to localhost:9000 as user default.
Connected to ClickHouse server version 20.10.1 revision 54440.

* `CREATE TABLE` statements
```
SHOW CREATE TABLE table0

┌─statement──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ CREATE TABLE default.table0
(
    `d` Date,
    `a` String,
    `b` UInt8,
    `x` String,
    `y` Int8,
    `version` UInt64,
    `sign` Int8 DEFAULT 1
)
ENGINE = ReplicatedVersionedCollapsingMergeTree('/clickhouse/tables/{shard}/table0', '{replica}', sign, version)
PARTITION BY y
ORDER BY d
SETTINGS index_granularity = 8192 │
└────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘

1 rows in set. Elapsed: 0.004 sec.
```

* Queries to run that lead to unexpected result
```
ALTER TABLE table0 ADD COLUMN order UInt32, MODIFY ORDER BY (d, order)
```
{
    {
        std::lock_guard lock(existing_nodes_cache_mutex);
        if (existing_nodes_cache.count(path))
            return true;
    }
    bool res = getZooKeeper()->exists(path);
    if (res)
    {
        std::lock_guard lock(existing_nodes_cache_mutex);
        existing_nodes_cache.insert(path);
    }
    return res;
}
std::optional<EphemeralLockInZooKeeper>
StorageReplicatedMergeTree::allocateBlockNumber(
    const String & partition_id, zkutil::ZooKeeperPtr & zookeeper, const String & zookeeper_block_id_path)
{
    
    Coordination::Requests deduplication_check_ops;
    if (!zookeeper_block_id_path.empty())
    {
        deduplication_check_ops.emplace_back(zkutil::makeCreateRequest(zookeeper_block_id_path, "", zkutil::CreateMode::Persistent));
        deduplication_check_ops.emplace_back(zkutil::makeRemoveRequest(zookeeper_block_id_path, -1));
    }
    String block_numbers_path = zookeeper_path + "/block_numbers";
    String partition_path = block_numbers_path + "/" + partition_id;
    if (!existsNodeCached(partition_path))
    {
        Coordination::Requests ops;