**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)
```
TableLockHolder table_lock_holder;
    if (!to_detached)
        table_lock_holder = lockForShare(RWLockImpl::NO_QUERY, getSettings()->lock_acquire_timeout_for_background_operations);
    
    Stopwatch stopwatch;
    MutableDataPartPtr part;
    DataPartsVector replaced_parts;
    auto write_part_log = [&] (const ExecutionStatus & execution_status)
    {
        writePartLog(
            PartLogElement::DOWNLOAD_PART, execution_status, stopwatch.elapsed(),
            part_name, part, replaced_parts, nullptr);
    };
    DataPartPtr part_to_clone;
    {
        
        
        
        auto covered_part_info = part_info;
        covered_part_info.mutation = 0;
        auto source_part = getActiveContainingPart(covered_part_info);
        if (source_part)
        {
            MinimalisticDataPartChecksums source_part_checksums;
            source_part_checksums.computeTotalChecksums(source_part->checksums);
            MinimalisticDataPartChecksums desired_checksums;
            String part_path = source_replica_path + "/parts/" + part_name;
            String part_znode = zookeeper->get(part_path);
            if (!part_znode.empty())
                desired_checksums = ReplicatedMergeTreePartHeader::fromString(part_znode).getChecksums();