**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)
```
+ toString(max_retries) + " tries");
            }
        }
        else
        {
            size_t try_no = 0;
            for (; try_no < max_retries; ++try_no)
            {
                std::lock_guard merge_selecting_lock(merge_selecting_mutex);
                ReplicatedMergeTreeMergePredicate can_merge = queue.getMergePredicate(zookeeper);
                FutureMergedMutatedPart future_merged_part;
                String disable_reason;
                bool selected = false;
                if (!partition)
                {
                    selected = merger_mutator.selectPartsToMerge(
                        future_merged_part, true, storage_settings_ptr->max_bytes_to_merge_at_max_space_in_pool, can_merge, false, &disable_reason);
                }
                else
                {
                    UInt64 disk_space = getStoragePolicy()->getMaxUnreservedFreeSpace();
                    String partition_id = getPartitionIDFromQuery(partition, query_context);
                    selected = merger_mutator.selectAllPartsToMergeWithinPartition(
                        future_merged_part, disk_space, can_merge, partition_id, final, &disable_reason);
                }
                if (!selected)
                {
                    std::stringstream message;
                    message << "Cannot select parts for optimization";
                    if (!disable_reason.empty())