**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)
```
const Strings & replicas, const String & mutation_id) const
{
    if (replicas.empty())
        return;
    zkutil::EventPtr wait_event = std::make_shared<Poco::Event>();
    std::set<String> inactive_replicas;
    for (const String & replica : replicas)
    {
        LOG_DEBUG(log, "Waiting for {} to apply mutation {}", replica, mutation_id);
        while (!partial_shutdown_called)
        {
            
            
            Coordination::Stat exists_stat;
            if (!getZooKeeper()->exists(zookeeper_path + "/mutations/" + mutation_id, &exists_stat, wait_event))
            {
                throw Exception(ErrorCodes::UNFINISHED, "Mutation {} was killed, manually removed or table was dropped", mutation_id);
            }
            auto zookeeper = getZooKeeper();
            
            if (!zookeeper->exists(zookeeper_path + "/replicas/" + replica + "/is_active"))
            {
                LOG_WARNING(log, "Replica {} is not active during mutation. Mutation will be done asynchronously when replica becomes active.", replica);
                inactive_replicas.emplace(replica);
                break;