**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)
```
*/
    unsigned try_no = 0;
    Strings missing_parts;
    do
    {
        if (try_no)
            LOG_INFO(log, "Some of parts ({}) are missing. Will try to fetch covering parts.", missing_parts.size());
        if (try_no >= query_context.getSettings().max_fetch_partition_retries_count)
            throw Exception("Too many retries to fetch parts from " + best_replica_path, ErrorCodes::TOO_MANY_RETRIES_TO_FETCH_PARTS);
        Strings parts = zookeeper->getChildren(best_replica_path + "/parts");
        ActiveDataPartSet active_parts_set(format_version, parts);
        Strings parts_to_fetch;
        if (missing_parts.empty())
        {
            parts_to_fetch = active_parts_set.getParts();
            
            Strings parts_to_fetch_partition;
            for (const String & part : parts_to_fetch)
            {
                if (MergeTreePartInfo::fromPartName(part, format_version).partition_id == partition_id)
                    parts_to_fetch_partition.push_back(part);
            }
            parts_to_fetch = std::move(parts_to_fetch_partition);
            if (parts_to_fetch.empty())
                throw Exception("Partition " + partition_id + " on " + best_replica_path + " doesn't exist", ErrorCodes::PARTITION_DOESNT_EXIST);
        }
        else
        {
            for (const String & missing_part : missing_parts)
            {