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.
}
        for (auto & block : block_infos)
        {
            Coordination::GetResponse resp = block.contents_future.get();
            if (resp.error == Coordination::Error::ZOK && lock_holder_paths.count(resp.data))
                committing_blocks[block.partition].insert(block.number);
        }
    }
    merges_version = queue_.pullLogsToQueue(zookeeper);
    Coordination::GetResponse quorum_status_response = quorum_status_future.get();
    if (quorum_status_response.error == Coordination::Error::ZOK)
    {
        ReplicatedMergeTreeQuorumEntry quorum_status;
        quorum_status.fromString(quorum_status_response.data);
        inprogress_quorum_part = quorum_status.part_name;
    }
    else
        inprogress_quorum_part.clear();
}
bool ReplicatedMergeTreeMergePredicate::operator()(
    const MergeTreeData::DataPartPtr & left,
    const MergeTreeData::DataPartPtr & right,
    String * out_reason) const
{
    if (left)
        return canMergeTwoParts(left, right, out_reason);
    else
        return canMergeSinglePart(right, out_reason);
}
bool ReplicatedMergeTreeMergePredicate::canMergeTwoParts(