**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)
```
{
    
    fetcher.blocker.cancelForever();
    merger_mutator.merges_blocker.cancelForever();
    parts_mover.moves_blocker.cancelForever();
    restarting_thread.shutdown();
    if (queue_task_handle)
        global_context.getBackgroundPool().removeTask(queue_task_handle);
    {
        
        
        auto lock = queue.lockQueue();
        queue_task_handle.reset();
        
        
        
        queue.pull_log_blocker.cancelForever();
    }
    if (move_parts_task_handle)
        global_context.getBackgroundMovePool().removeTask(move_parts_task_handle);
    move_parts_task_handle.reset();
    if (data_parts_exchange_endpoint)
    {
        global_context.getInterserverIOHandler().removeEndpointIfExists(data_parts_exchange_endpoint->getId(replica_path));
        
        data_parts_exchange_endpoint->blocker.cancelForever();
        
        std::unique_lock lock(data_parts_exchange_endpoint->rwlock);