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.
alter_sequence.finishMetadataAlter(entry->alter_version, state_lock);
        }
    }
    else
    {
        for (const String & virtual_part_name : entry->getVirtualPartNames())
        {
            
            
            
            removeCoveredPartsFromMutations(virtual_part_name,  true,  false);
        }
    }
}
void ReplicatedMergeTreeQueue::removeCoveredPartsFromMutations(const String & part_name, bool remove_part, bool remove_covered_parts)
{
    auto part_info = MergeTreePartInfo::fromPartName(part_name, format_version);
    auto in_partition = mutations_by_partition.find(part_info.partition_id);
    if (in_partition == mutations_by_partition.end())
        return;
    bool some_mutations_are_probably_done = false;
    auto from_it = in_partition->second.lower_bound(part_info.getDataVersion());
    for (auto it = from_it; it != in_partition->second.end(); ++it)
    {
        MutationStatus & status = *it->second;
        if (remove_part && remove_covered_parts)
            status.parts_to_do.removePartAndCoveredParts(part_name);
        else if (remove_covered_parts)
            status.parts_to_do.removePartsCoveredBy(part_name);
        else if (remove_part)