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.
if (left->info.partition_id != right->info.partition_id)
    {
        if (out_reason)
            *out_reason = "Parts " + left->name + " and " + right->name + " belong to different partitions";
        return false;
    }
    for (const MergeTreeData::DataPartPtr & part : {left, right})
    {
        if (part->name == inprogress_quorum_part)
        {
            if (out_reason)
                *out_reason = "Quorum insert for part " + part->name + " is currently in progress";
            return false;
        }
        if (prev_virtual_parts.getContainingPart(part->info).empty())
        {
            if (out_reason)
                *out_reason = "Entry for part " + part->name + " hasn't been read from the replication log yet";
            return false;
        }
    }
    Int64 left_max_block = left->info.max_block;
    Int64 right_min_block = right->info.min_block;
    if (left_max_block > right_min_block)