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.
{
    LogEntryPtr entry;
    std::lock_guard lock(state_mutex);
    for (auto it = queue.begin(); it != queue.end(); ++it)
    {
        if ((*it)->currently_executing)
            continue;
        if (shouldExecuteLogEntry(**it, (*it)->postpone_reason, merger_mutator, data, lock))
        {
            entry = *it;
            
            
            queue.splice(queue.end(), queue, it);
            break;
        }
        else
        {
            ++(*it)->num_postponed;
            (*it)->last_postpone_time = time(nullptr);
        }
    }
    if (entry)
        return std::make_shared<SelectedEntry>(entry, std::unique_ptr<CurrentlyExecuting>{ new CurrentlyExecuting(entry, *this) });
    else
        return {};
}
bool ReplicatedMergeTreeQueue::processEntry(