**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)
```
ReplicatedMergeTreeMutationEntry entry;
    entry.source_replica = replica_name;
    entry.commands = commands;
    String mutations_path = zookeeper_path + "/mutations";
    
    
    
    while (true)
    {
        auto zookeeper = getZooKeeper();
        Coordination::Stat mutations_stat;
        zookeeper->get(mutations_path, &mutations_stat);
        EphemeralLocksInAllPartitions block_number_locks(
            zookeeper_path + "/block_numbers", "block-", zookeeper_path + "/temp", *zookeeper);
        for (const auto & lock : block_number_locks.getLocks())
            entry.block_numbers[lock.partition_id] = lock.number;
        entry.create_time = time(nullptr);
        Coordination::Requests requests;
        requests.emplace_back(zkutil::makeSetRequest(mutations_path, String(), mutations_stat.version));
        requests.emplace_back(zkutil::makeCreateRequest(
            mutations_path + "/", entry.toString(), zkutil::CreateMode::PersistentSequential));
        Coordination::Responses responses;
        Coordination::Error rc = zookeeper->tryMulti(requests, responses);
        if (rc == Coordination::Error::ZOK)
        {