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.
zkutil::ZooKeeperPtr zookeeper, const String & mutation_id)
{
    std::lock_guard lock(update_mutations_mutex);
    auto rc = zookeeper->tryRemove(zookeeper_path + "/mutations/" + mutation_id);
    if (rc == Coordination::Error::ZOK)
        LOG_DEBUG(log, "Removed mutation {} from ZooKeeper.", mutation_id);
    ReplicatedMergeTreeMutationEntryPtr entry;
    bool mutation_was_active = false;
    {
        std::lock_guard state_lock(state_mutex);
        auto it = mutations_by_znode.find(mutation_id);
        if (it == mutations_by_znode.end())