**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)
```
{
    size_t num_tries = 0;
    bool success = false;
    while (!success && (max_retries == 0 || num_tries < max_retries))
    {
        try
        {
            ++num_tries;
            success = true;
            auto zookeeper = getZooKeeper();
            std::vector<std::future<Coordination::ExistsResponse>> exists_futures;
            exists_futures.reserve(part_names.size());
            for (const String & part_name : part_names)
            {
                String part_path = replica_path + "/parts/" + part_name;
                exists_futures.emplace_back(zookeeper->asyncExists(part_path));
            }
            std::vector<std::future<Coordination::MultiResponse>> remove_futures;
            remove_futures.reserve(part_names.size());
            for (size_t i = 0; i < part_names.size(); ++i)
            {
                Coordination::ExistsResponse exists_resp = exists_futures[i].get();
                if (exists_resp.error == Coordination::Error::ZOK)
                {
                    Coordination::Requests ops;
                    removePartFromZooKeeper(part_names[i], ops, exists_resp.stat.numChildren > 0);