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(queue.subscribers_mutex);
    queue.subscribers.erase(it);
}
void ReplicatedMergeTreeQueue::notifySubscribers(size_t new_queue_size)
{
    std::lock_guard lock_subscribers(subscribers_mutex);
    for (auto & subscriber_callback : subscribers)
        subscriber_callback(new_queue_size);
}
ReplicatedMergeTreeQueue::~ReplicatedMergeTreeQueue()
{
    notifySubscribers(0);
}
String padIndex(Int64 index)
{
    String index_str = toString(index);
    return std::string(10 - index_str.size(), '0') + index_str;
}
void ReplicatedMergeTreeQueue::removeCurrentPartsFromMutations()
{
    std::lock_guard state_lock(state_mutex);
    for (const auto & part_name : current_parts.getParts())
        removeCoveredPartsFromMutations(part_name,  true,  true);
}
}