**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)
```
LOG_WARNING(log, "Some (but not all) parts from REPLACE PARTITION command already exist. REPLACE PARTITION will not be atomic.");
    }
    StoragePtr source_table;
    TableLockHolder table_lock_holder_src_table;
    StorageID source_table_id{entry_replace.from_database, entry_replace.from_table};
    auto clone_data_parts_from_source_table = [&] () -> size_t
    {
        source_table = DatabaseCatalog::instance().tryGetTable(source_table_id, global_context);
        if (!source_table)
        {
            LOG_DEBUG(log, "Can't use {} as source table for REPLACE PARTITION command. It does not exist.", source_table_id.getNameForLogs());
            return 0;
        }
        auto src_metadata_snapshot = source_table->getInMemoryMetadataPtr();
        MergeTreeData * src_data = nullptr;
        try
        {
            src_data = &checkStructureAndGetMergeTreeData(source_table, src_metadata_snapshot, dst_metadata_snapshot);
        }
        catch (Exception &)
        {
            LOG_INFO(log, "Can't use {} as source table for REPLACE PARTITION command. Will fetch all parts. Reason: {}", source_table_id.getNameForLogs(), getCurrentExceptionMessage(false));
            return 0;
        }
        table_lock_holder_src_table = source_table->lockForShare(
                RWLockImpl::NO_QUERY, getSettings()->lock_acquire_timeout_for_background_operations);
        DataPartStates valid_states{
            MergeTreeDataPartState::PreCommitted, MergeTreeDataPartState::Committed, MergeTreeDataPartState::Outdated};
        size_t num_clonable_parts = 0;
        for (PartDescriptionPtr & part_desc : parts_to_add)