**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)
```
partition_ids.emplace(part->info.partition_id);
            UInt64 disk_space = getStoragePolicy()->getMaxUnreservedFreeSpace();
            for (const String & partition_id : partition_ids)
            {
                size_t try_no = 0;
                for (; try_no < max_retries; ++try_no)
                {
                    
                    
                    std::lock_guard merge_selecting_lock(merge_selecting_mutex);
                    ReplicatedMergeTreeMergePredicate can_merge = queue.getMergePredicate(zookeeper);
                    FutureMergedMutatedPart future_merged_part;
                    bool selected = merger_mutator.selectAllPartsToMergeWithinPartition(
                        future_merged_part, disk_space, can_merge, partition_id, true, nullptr);
                    if (!selected)
                        break;
                    ReplicatedMergeTreeLogEntryData merge_entry;
                    CreateMergeEntryResult create_result = createLogEntryToMergeParts(
                        zookeeper, future_merged_part.parts,
                        future_merged_part.name, future_merged_part.type, deduplicate,
                        &merge_entry, can_merge.getVersion(), future_merged_part.merge_type);
                    if (create_result == CreateMergeEntryResult::MissingPart)
                        return handle_noop("Can't create merge queue node in ZooKeeper, because some parts are missing");
                    if (create_result == CreateMergeEntryResult::LogUpdated)
                        continue;
                    merge_entries.push_back(std::move(merge_entry));
                    break;
                }
                if (try_no == max_retries)
                    return handle_noop("Can't create merge queue node in ZooKeeper, because log was updated in every of "