**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)
```
Coordination::Requests ops;
    Coordination::Responses responses;
    ops.emplace_back(zkutil::makeCreateRequest(
        zookeeper_path + "/log/log-", entry.toString(),
        zkutil::CreateMode::PersistentSequential));
    ops.emplace_back(zkutil::makeSetRequest(
        zookeeper_path + "/log", "", log_version)); 
    Coordination::Error code = zookeeper->tryMulti(ops, responses);
    if (code == Coordination::Error::ZOK)
    {
        String path_created = dynamic_cast<const Coordination::CreateResponse &>(*responses.front()).path_created;
        entry.znode_name = path_created.substr(path_created.find_last_of('/') + 1);
        ProfileEvents::increment(ProfileEvents::CreatedLogEntryForMerge);
        LOG_TRACE(log, "Created log entry {} for merge {}", path_created, merged_name);
    }
    else if (code == Coordination::Error::ZBADVERSION)
    {
        ProfileEvents::increment(ProfileEvents::NotCreatedLogEntryForMerge);
        LOG_TRACE(log, "Log entry is not created for merge {} because log was updated", merged_name);
        return CreateMergeEntryResult::LogUpdated;
    }
    else
    {
        zkutil::KeeperMultiException::check(code, ops, responses);
    }
    if (out_log_entry)
        *out_log_entry = entry;
    return CreateMergeEntryResult::Ok;
}
StorageReplicatedMergeTree::CreateMergeEntryResult StorageReplicatedMergeTree::createLogEntryToMutatePart(