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 LogEntryPtr & entry, std::optional<time_t> & min_unprocessed_insert_time_changed,
    std::lock_guard<std::mutex> & state_lock)
{
    for (const String & virtual_part_name : entry->getVirtualPartNames())
    {
        virtual_parts.add(virtual_part_name);
        addPartToMutations(virtual_part_name);
    }
    
    if (entry->type != LogEntry::DROP_RANGE)
        queue.push_back(entry);
    else
        queue.push_front(entry);
    if (entry->type == LogEntry::GET_PART)
    {
        inserts_by_time.insert(entry);
        if (entry->create_time && (!min_unprocessed_insert_time || entry->create_time < min_unprocessed_insert_time))
        {
            min_unprocessed_insert_time = entry->create_time;
            min_unprocessed_insert_time_changed = min_unprocessed_insert_time;
        }
    }
    if (entry->type == LogEntry::ALTER_METADATA)
    {
        LOG_TRACE(log, "Adding alter metadata version {} to the queue", entry->alter_version);
        alter_sequence.addMetadataAlter(entry->alter_version, entry->have_mutation, state_lock);
    }
}
void ReplicatedMergeTreeQueue::insert(zkutil::ZooKeeperPtr zookeeper, LogEntryPtr & entry)