**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)
```
{
            Coordination::Stat columns_stat_before, columns_stat_after;
            String columns_str;
            String checksums_str;
            
            
            if (!zookeeper->tryGet(current_part_path + "/columns", columns_str, &columns_stat_before) ||
                !zookeeper->tryGet(current_part_path + "/checksums", checksums_str) ||
                !zookeeper->exists(current_part_path + "/columns", &columns_stat_after) ||
                columns_stat_before.version != columns_stat_after.version)
            {
                LOG_INFO(log, "Not checking checksums of part {} with replica {} because part changed while we were reading its checksums", part_name, replica);
                continue;
            }
            replica_part_header = ReplicatedMergeTreePartHeader::fromColumnsAndChecksumsZNodes(
                columns_str, checksums_str);
        }
        if (replica_part_header.getColumnsHash() != local_part_header.getColumnsHash())
        {
            LOG_INFO(log, "Not checking checksums of part {} with replica {} because columns are different", part_name, replica);
            continue;
        }
        replica_part_header.getChecksums().checkEqual(local_part_header.getChecksums(), true);
        if (replica == replica_name)
            has_been_already_added = true;
        
        
        if (absent_replicas_paths)
        {
            absent_replicas_paths->clear();