**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)
```
const IMergeTreeDataPart & part, Int64 mutation_version, int32_t alter_version, int32_t log_version)
{
    auto zookeeper = getZooKeeper();
    
    if (!zookeeper->exists(replica_path + "/parts/" + part.name))
    {
        if (part.modification_time + MAX_AGE_OF_LOCAL_PART_THAT_WASNT_ADDED_TO_ZOOKEEPER < time(nullptr))
        {
            LOG_WARNING(log, "Part {} (that was selected for mutation) with age {} seconds exists locally but not in ZooKeeper."
                " Won't mutate that part and will check it.", part.name, (time(nullptr) - part.modification_time));
            enqueuePartForCheck(part.name);
        }
        return CreateMergeEntryResult::MissingPart;
    }
    MergeTreePartInfo new_part_info = part.info;
    new_part_info.mutation = mutation_version;
    String new_part_name = part.getNewName(new_part_info);
    ReplicatedMergeTreeLogEntryData entry;
    entry.type = LogEntry::MUTATE_PART;
    entry.source_replica = replica_name;
    entry.source_parts.push_back(part.name);
    entry.new_part_name = new_part_name;
    entry.create_time = time(nullptr);
    entry.alter_version = alter_version;