**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)
```
const String & zookeeper_path_,
    const String & replica_name_,
    bool attach,
    const StorageID & table_id_,
    const String & relative_data_path_,
    const StorageInMemoryMetadata & metadata_,
    Context & context_,
    const String & date_column_name,
    const MergingParams & merging_params_,
    std::unique_ptr<MergeTreeSettings> settings_,
    bool has_force_restore_data_flag,
    bool allow_renaming_)
    : MergeTreeData(table_id_,
                    relative_data_path_,
                    metadata_,
                    context_,
                    date_column_name,
                    merging_params_,
                    std::move(settings_),
                    true,                   
                    attach,
                    [this] (const std::string & name) { enqueuePartForCheck(name); })
    , zookeeper_path(normalizeZooKeeperPath(zookeeper_path_))
    , replica_name(replica_name_)
    , replica_path(zookeeper_path + "/replicas/" + replica_name)
    , reader(*this)
    , writer(*this)
    , merger_mutator(*this, global_context.getBackgroundPool().getNumberOfThreads())
    , queue(*this)
    , fetcher(*this)