**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)
```
Stopwatch stopwatch;
    auto write_part_log = [&] (const ExecutionStatus & execution_status)
    {
        writePartLog(
            PartLogElement::MUTATE_PART, execution_status, stopwatch.elapsed(),
            entry.new_part_name, new_part, future_mutated_part.parts, merge_entry.get());
    };
    try
    {
        new_part = merger_mutator.mutatePartToTemporaryPart(
            future_mutated_part, metadata_snapshot, commands, *merge_entry,
            entry.create_time, global_context, reserved_space, table_lock);
        renameTempPartAndReplace(new_part, nullptr, &transaction);
        try
        {
            checkPartChecksumsAndCommit(transaction, new_part);
        }
        catch (const Exception & e)
        {
            if (MergeTreeDataPartChecksums::isBadChecksumsErrorCode(e.code()))
            {
                transaction.rollback();
                ProfileEvents::increment(ProfileEvents::DataAfterMutationDiffersFromReplica);
                LOG_ERROR(log, "{}. Data after mutation is not byte-identical to data on another replicas. We will download merged part from replica to force byte-identical result.", getCurrentExceptionMessage(false));
                write_part_log(ExecutionStatus::fromCurrentException());
                tryRemovePartImmediately(std::move(new_part));
                
                
                return false;
            }