**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)
```
zkutil::ZooKeeper & zookeeper, String & partition_id, StorageReplicatedMergeTree::LogEntry & entry, bool detach)
{
    MergeTreePartInfo drop_range_info;
    if (!getFakePartCoveringAllPartsInPartition(partition_id, drop_range_info))
    {
        LOG_INFO(log, "Will not drop partition {}, it is empty.", partition_id);
        return false;
    }
    clearBlocksInPartition(zookeeper, partition_id, drop_range_info.min_block, drop_range_info.max_block);
    /** Forbid to choose the parts to be deleted for merging.
      * Invariant: after the `DROP_RANGE` entry appears in the log, merge of deleted parts will not appear in the log.
      */
    String drop_range_fake_part_name = getPartNamePossiblyFake(format_version, drop_range_info);
    {
        std::lock_guard merge_selecting_lock(merge_selecting_mutex);
        queue.disableMergesInBlockRange(drop_range_fake_part_name);
    }
    LOG_DEBUG(log, "Disabled merges covered by range {}", drop_range_fake_part_name);
    
    entry.type = LogEntry::DROP_RANGE;
    entry.source_replica = replica_name;
    entry.new_part_name = drop_range_fake_part_name;
    entry.detach = detach;
    entry.create_time = time(nullptr);
    Coordination::Requests ops;
    ops.emplace_back(zkutil::makeCreateRequest(zookeeper_path + "/log/log-", entry.toString(), zkutil::CreateMode::PersistentSequential));
    ops.emplace_back(zkutil::makeSetRequest(zookeeper_path + "/log", "", -1));