**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 (merges_and_mutations_sum >= storage_settings_ptr->max_replicated_merges_in_queue)
        {
            LOG_TRACE(log, "Number of queued merges ({}) and part mutations ({})"
                " is greater than max_replicated_merges_in_queue ({}), so won't select new parts to merge or mutate.",
                merges_and_mutations_queued.merges,
                merges_and_mutations_queued.mutations,
                storage_settings_ptr->max_replicated_merges_in_queue);
        }
        else
        {
            UInt64 max_source_parts_size_for_merge = merger_mutator.getMaxSourcePartsSizeForMerge(
                storage_settings_ptr->max_replicated_merges_in_queue, merges_and_mutations_sum);
            UInt64 max_source_part_size_for_mutation = merger_mutator.getMaxSourcePartSizeForMutation();
            bool merge_with_ttl_allowed = merges_and_mutations_queued.merges_with_ttl < storage_settings_ptr->max_replicated_merges_with_ttl_in_queue &&
                getTotalMergesWithTTLInMergeList() < storage_settings_ptr->max_number_of_merges_with_ttl_in_pool;
            FutureMergedMutatedPart future_merged_part;
            if (max_source_parts_size_for_merge > 0 &&
                merger_mutator.selectPartsToMerge(future_merged_part, false, max_source_parts_size_for_merge, merge_pred, merge_with_ttl_allowed, nullptr))
            {
                create_result = createLogEntryToMergeParts(zookeeper, future_merged_part.parts,
                    future_merged_part.name, future_merged_part.type, deduplicate, nullptr, merge_pred.getVersion(), future_merged_part.merge_type);
            }
            
            else if (max_source_part_size_for_mutation > 0 && queue.countMutations() > 0
                     && merges_and_mutations_queued.mutations < storage_settings_ptr->max_replicated_mutations_in_queue)
            {
                
                DataPartsVector data_parts = getDataPartsVector();
                for (const auto & part : data_parts)
                {