**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)
```
{
    UInt64 res = 0;
    foreachCommittedParts([&res](auto & part) { res += part->rows_count; });
    return res;
}
std::optional<UInt64> StorageReplicatedMergeTree::totalBytes() const
{
    UInt64 res = 0;
    foreachCommittedParts([&res](auto & part) { res += part->getBytesOnDisk(); });
    return res;
}
void StorageReplicatedMergeTree::assertNotReadonly() const
{
    if (is_readonly)
        throw Exception(ErrorCodes::TABLE_IS_READ_ONLY, "Table is in readonly mode (zookeeper path: {})", zookeeper_path);
}
BlockOutputStreamPtr StorageReplicatedMergeTree::write(const ASTPtr & , const StorageMetadataPtr & metadata_snapshot, const Context & context)
{
    const auto storage_settings_ptr = getSettings();
    assertNotReadonly();
    const Settings & query_settings = context.getSettingsRef();
    bool deduplicate = storage_settings_ptr->replicated_deduplication_window != 0 && query_settings.insert_deduplicate;
    return std::make_shared<ReplicatedMergeTreeBlockOutputStream>(
        *this, metadata_snapshot, query_settings.insert_quorum,
        query_settings.insert_quorum_timeout.totalMilliseconds(),
        query_settings.max_partitions_per_insert_block,
        query_settings.insert_quorum_parallel,
        deduplicate);
}
bool StorageReplicatedMergeTree::optimize(