**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)
```
{
    auto zookeeper = getZooKeeper();
    ReplicatedMergeTreeTableMetadata old_metadata(*this, metadata_snapshot);
    Coordination::Stat metadata_stat;
    String metadata_str = zookeeper->get(zookeeper_prefix + "/metadata", &metadata_stat);
    auto metadata_from_zk = ReplicatedMergeTreeTableMetadata::parse(metadata_str);
    old_metadata.checkEquals(metadata_from_zk, metadata_snapshot->getColumns(), global_context);
    Coordination::Stat columns_stat;
    auto columns_from_zk = ColumnsDescription::parse(zookeeper->get(zookeeper_prefix + "/columns", &columns_stat));
    const ColumnsDescription & old_columns = metadata_snapshot->getColumns();
    if (columns_from_zk != old_columns)
    {
        throw Exception("Table columns structure in ZooKeeper is different from local table structure", ErrorCodes::INCOMPATIBLE_COLUMNS);
    }
}
void StorageReplicatedMergeTree::setTableStructure(