**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)
```
}
        }
        ops.emplace_back(zkutil::makeSetRequest(zookeeper_path + "/log", "", -1));  
        ops.emplace_back(zkutil::makeCreateRequest(zookeeper_path + "/log/log-", entry.toString(), zkutil::CreateMode::PersistentSequential));
        Transaction transaction(*this);
        {
            auto data_parts_lock = lockParts();
            for (MutableDataPartPtr & part : dst_parts)
                renameTempPartAndReplace(part, nullptr, &transaction, data_parts_lock);
        }
        op_results = zookeeper->multi(ops);
        {
            auto data_parts_lock = lockParts();
            transaction.commit(&data_parts_lock);
            if (replace)
                parts_to_remove = removePartsInRangeFromWorkingSet(drop_range, true, false, data_parts_lock);
        }
        PartLog::addNewParts(global_context, dst_parts, watch.elapsed());
    }
    catch (...)
    {
        PartLog::addNewParts(global_context, dst_parts, watch.elapsed(), ExecutionStatus::fromCurrentException());
        throw;
    }
    String log_znode_path = dynamic_cast<const Coordination::CreateResponse &>(*op_results.back()).path_created;
    entry.znode_name = log_znode_path.substr(log_znode_path.find_last_of('/') + 1);
    for (auto & lock : ephemeral_locks)
        lock.assumeUnlocked();
    
    tryRemovePartsFromZooKeeperWithRetries(parts_to_remove);