**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)
```
{
            auto src_part = src_data->getPartIfExists(part_desc->src_part_info, valid_states);
            if (!src_part)
            {
                LOG_DEBUG(log, "There is no part {} in {}", part_desc->src_part_name, source_table_id.getNameForLogs());
                continue;
            }
            String checksum_hex  = src_part->checksums.getTotalChecksumHex();
            if (checksum_hex != part_desc->checksum_hex)
            {
                LOG_DEBUG(log, "Part {} of {} has inappropriate checksum", part_desc->src_part_name, source_table_id.getNameForLogs());
                
                continue;
            }
            part_desc->found_new_part_name = part_desc->new_part_name;
            part_desc->found_new_part_info = part_desc->new_part_info;
            part_desc->src_table_part = src_part;
            ++num_clonable_parts;
        }
        return num_clonable_parts;
    };
    size_t num_clonable_parts = clone_data_parts_from_source_table();
    LOG_DEBUG(log, "Found {} parts that could be cloned (of {} required parts)", num_clonable_parts, parts_to_add.size());
    ActiveDataPartSet adding_parts_active_set(format_version);
    std::unordered_map<String, PartDescriptionPtr> part_name_to_desc;
    for (PartDescriptionPtr & part_desc : parts_to_add)
    {
        if (part_desc->src_table_part)
        {