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.
virtual_parts.remove(part_name);
        for (Queue::iterator it = queue.begin(); it != queue.end();)
        {
            if ((*it)->new_part_name == part_name)
            {
                found = *it;
                updateStateOnQueueEntryRemoval(
                    found,  false,
                    min_unprocessed_insert_time_changed, max_processed_insert_time_changed, lock);
                queue.erase(it++);
                queue_size = queue.size();
                break;
            }
            else
                ++it;
        }
    }
    if (!found)
        return false;
    notifySubscribers(queue_size);
    zookeeper->tryRemove(replica_path + "/queue/" + found->znode_name);
    updateTimesInZooKeeper(zookeeper, min_unprocessed_insert_time_changed, max_processed_insert_time_changed);
    return true;
}
bool ReplicatedMergeTreeQueue::removeFromVirtualParts(const MergeTreePartInfo & part_info)
{
    std::lock_guard lock(state_mutex);
    return virtual_parts.remove(part_info);
}
int32_t ReplicatedMergeTreeQueue::pullLogsToQueue(zkutil::ZooKeeperPtr zookeeper, Coordination::WatchCallback watch_callback)