**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)
```
{
    if (entry.type == LogEntry::DROP_RANGE)
    {
        executeDropRange(entry);
        return true;
    }
    if (entry.type == LogEntry::REPLACE_RANGE)
    {
        executeReplaceRange(entry);
        return true;
    }
    if (entry.type == LogEntry::GET_PART ||
        entry.type == LogEntry::MERGE_PARTS ||
        entry.type == LogEntry::MUTATE_PART)
    {
        
        
        
        DataPartPtr existing_part = getPartIfExists(entry.new_part_name, {MergeTreeDataPartState::PreCommitted});
        if (!existing_part)
            existing_part = getActiveContainingPart(entry.new_part_name);
        
        if (existing_part && getZooKeeper()->exists(replica_path + "/parts/" + existing_part->name))
        {
            if (!(entry.type == LogEntry::GET_PART && entry.source_replica == replica_name))
            {
                LOG_DEBUG(log, "Skipping action for part {} because part {} already exists.", entry.new_part_name, existing_part->name);