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.
{
    std::lock_guard lock(state_mutex);
    size_t count_merges = 0;
    size_t count_mutations = 0;
    size_t count_merges_with_ttl = 0;
    for (const auto & entry : queue)
    {
        if (entry->type == ReplicatedMergeTreeLogEntry::MERGE_PARTS)
        {
            ++count_merges;
            if (isTTLMergeType(entry->merge_type))
                ++count_merges_with_ttl;
        }
        else if (entry->type == ReplicatedMergeTreeLogEntry::MUTATE_PART)
            ++count_mutations;
    }
    return OperationsInQueue{count_merges, count_mutations, count_merges_with_ttl};
}
size_t ReplicatedMergeTreeQueue::countMutations() const
{
    std::lock_guard lock(state_mutex);
    return mutations_by_znode.size();
}
size_t ReplicatedMergeTreeQueue::countFinishedMutations() const