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.
const String & partition_id, Int64 data_version, std::lock_guard<std::mutex> & ) const
{
    auto in_partition = mutations_by_partition.find(partition_id);
    if (in_partition == mutations_by_partition.end())
        return 0;
    auto it = in_partition->second.upper_bound(data_version);
    if (it == in_partition->second.begin())
        return 0;
    --it;
    return it->first;
}
Int64 ReplicatedMergeTreeQueue::getCurrentMutationVersion(const String & partition_id, Int64 data_version) const
{
    std::lock_guard lock(state_mutex);
    return getCurrentMutationVersionImpl(partition_id, data_version, lock);
}
ReplicatedMergeTreeQueue::CurrentlyExecuting::CurrentlyExecuting(const ReplicatedMergeTreeQueue::LogEntryPtr & entry_, ReplicatedMergeTreeQueue & queue_)