One of our users just killed mutations produced by `ALTER MODIFY` queries on 21 servers. After that new alters on three servers stuck with:
```
Cannot execute alter metadata with version: 8 because another alter 7 must be executed before
```

However, there is no corresponding alter №7 in the replication queue and no undone mutations:
```
select * from system.mutations where is_done = 0
```

However, in the ZooKeeper, we have the correct state of metadata_version:
```
Row 13:
───────
name:           metadata_version
value:          7
czxid:          23078663291
mzxid:          29401964979
ctime:          2020-09-02 17:52:08
mtime:          2020-11-13 00:43:18
version:        7
cversion:       0
aversion:       0
ephemeralOwner: 0
dataLength:     1
numChildren:    0
pzxid:          23078663291
```
`SYSTEM RESTART REPLICA dbname.table_name` solves the issue, but it seems like` KILL MUTATION` in some cases can lead to unfinished alter.
const ReplicatedMergeTreeMutationEntry & mutation, const ActiveDataPartSet & parts)
{
    Names result;
    for (const auto & pair : mutation.block_numbers)
    {
        const String & partition_id = pair.first;
        Int64 block_num = pair.second;
        
        
        MergeTreePartInfo covering_part_info(
            partition_id, 0, block_num, MergeTreePartInfo::MAX_LEVEL, MergeTreePartInfo::MAX_BLOCK_NUMBER);
        for (const String & covered_part_name : parts.getPartsCoveredBy(covering_part_info))
        {
            auto part_info = MergeTreePartInfo::fromPartName(covered_part_name, parts.getFormatVersion());
            if (part_info.getDataVersion() < block_num)
                result.push_back(covered_part_name);
        }
    }
    return result;
}
}
void ReplicatedMergeTreeQueue::updateMutations(zkutil::ZooKeeperPtr zookeeper, Coordination::WatchCallback watch_callback)