**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(current_zookeeper_mutex);
    current_zookeeper = zookeeper;
}
zkutil::ZooKeeperPtr StorageReplicatedMergeTree::tryGetZooKeeper() const
{
    std::lock_guard lock(current_zookeeper_mutex);
    return current_zookeeper;
}
zkutil::ZooKeeperPtr StorageReplicatedMergeTree::getZooKeeper() const
{
    auto res = tryGetZooKeeper();
    if (!res)
        throw Exception("Cannot get ZooKeeper", ErrorCodes::NO_ZOOKEEPER);
    return res;
}
static std::string normalizeZooKeeperPath(std::string zookeeper_path)
{
    if (!zookeeper_path.empty() && zookeeper_path.back() == '/')
        zookeeper_path.resize(zookeeper_path.size() - 1);
    
    if (!zookeeper_path.empty() && zookeeper_path.front() != '/')
        zookeeper_path = "/" + zookeeper_path;
    return zookeeper_path;
}
StorageReplicatedMergeTree::StorageReplicatedMergeTree(