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.
{
    res.clear();
    std::lock_guard lock(state_mutex);
    res.reserve(queue.size());
    for (const auto & entry : queue)
        res.emplace_back(*entry);
}
void ReplicatedMergeTreeQueue::getInsertTimes(time_t & out_min_unprocessed_insert_time, time_t & out_max_processed_insert_time) const
{
    std::lock_guard lock(state_mutex);
    out_min_unprocessed_insert_time = min_unprocessed_insert_time;
    out_max_processed_insert_time = max_processed_insert_time;
}
std::optional<MergeTreeMutationStatus> ReplicatedMergeTreeQueue::getIncompleteMutationsStatus(const String & znode_name, std::set<String> * mutation_ids) const
{
    std::lock_guard lock(state_mutex);
    auto current_mutation_it = mutations_by_znode.find(znode_name);