**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)
```
*  and it would be nice to be able to choose the replica closest by network.
        * NOTE Of course, there are data races here. You can solve it by retrying.
        */
        Int64 max_log_pointer = -1;
        UInt64 min_queue_size = std::numeric_limits<UInt64>::max();
        for (const String & replica : active_replicas)
        {
            String current_replica_path = from + "/replicas/" + replica;
            String log_pointer_str = zookeeper->get(current_replica_path + "/log_pointer");
            Int64 log_pointer = log_pointer_str.empty() ? 0 : parse<UInt64>(log_pointer_str);
            Coordination::Stat stat;
            zookeeper->get(current_replica_path + "/queue", &stat);
            size_t queue_size = stat.numChildren;
            if (log_pointer > max_log_pointer
                || (log_pointer == max_log_pointer && queue_size < min_queue_size))
            {
                max_log_pointer = log_pointer;
                min_queue_size = queue_size;
                best_replica = replica;
            }
        }
    }
    if (best_replica.empty())
        throw Exception("Logical error: cannot choose best replica.", ErrorCodes::LOGICAL_ERROR);
    LOG_INFO(log, "Found {} replicas, {} of them are active. Selected {} to fetch from.", replicas.size(), active_replicas.size(), best_replica);
    String best_replica_path = from + "/replicas/" + best_replica;
    
    /** Trying to download these parts.
      * Some of them could be deleted due to the merge.
      * In this case, update the information about the available parts and try again.