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::optional<time_t> min_unprocessed_insert_time_changed;
    std::optional<time_t> max_processed_insert_time_changed;
    bool found = false;
    bool need_remove_from_zk = true;
    size_t queue_size = 0;
    
    {
        std::unique_lock lock(state_mutex);
        if (entry->removed_by_other_entry)
        {
            need_remove_from_zk = false;
            queue_size = queue.size();
        }
        else
        {
            
            
            
            
            for (Queue::iterator it = queue.end(); it != queue.begin();)
            {
                --it;
                if (*it == entry)