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.state_mutex);
    if (entry->type == ReplicatedMergeTreeLogEntry::DROP_RANGE || entry->type == ReplicatedMergeTreeLogEntry::REPLACE_RANGE)
    {
        assert(queue.currently_executing_drop_or_replace_range);
        queue.currently_executing_drop_or_replace_range = false;
    }
    entry->currently_executing = false;
    entry->execution_complete.notify_all();
    for (const String & new_part_name : entry->getBlockingPartNames())
    {
        if (!queue.future_parts.erase(new_part_name))
            LOG_ERROR(queue.log, "Untagging already untagged future part {}. This is a bug.", new_part_name);
    }
    if (!entry->actual_new_part_name.empty())
    {
        if (entry->actual_new_part_name != entry->new_part_name && !queue.future_parts.erase(entry->actual_new_part_name))
            LOG_ERROR(queue.log, "Untagging already untagged future part {}. This is a bug.", entry->actual_new_part_name);
        entry->actual_new_part_name.clear();
    }
}
ReplicatedMergeTreeQueue::SelectedEntryPtr ReplicatedMergeTreeQueue::selectEntryToProcess(MergeTreeDataMergerMutator & merger_mutator, MergeTreeData & data)