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 = 0;
    for (const auto & pair : mutations_by_znode)
    {
        const auto & mutation = pair.second;
        if (!mutation.is_done)
            break;
        ++count;
    }
    return count;
}
ReplicatedMergeTreeMergePredicate ReplicatedMergeTreeQueue::getMergePredicate(zkutil::ZooKeeperPtr & zookeeper)
{
    return ReplicatedMergeTreeMergePredicate(*this, zookeeper);
}
MutationCommands ReplicatedMergeTreeQueue::getFirstAlterMutationCommandsForPart(const MergeTreeData::DataPartPtr & part) const
{
    std::lock_guard lock(state_mutex);
    auto in_partition = mutations_by_partition.find(part->info.partition_id);
    if (in_partition == mutations_by_partition.end())
        return MutationCommands{};
    Int64 part_version = part->info.getDataVersion();
    for (auto [mutation_version, mutation_status] : in_partition->second)
        if (mutation_version > part_version && mutation_status->entry->alter_version != -1)
            return mutation_status->entry->commands;
    return MutationCommands{};
}
MutationCommands ReplicatedMergeTreeQueue::getMutationCommands(