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.
: storage(storage_)
    , merge_strategy_picker(merge_strategy_picker_)
    , format_version(storage.format_version)
    , current_parts(format_version)
    , virtual_parts(format_version)
{
    zookeeper_path = storage.zookeeper_path;
    replica_path = storage.replica_path;
    logger_name = storage.getStorageID().getFullTableName() + " (ReplicatedMergeTreeQueue)";
    log = &Poco::Logger::get(logger_name);
}
void ReplicatedMergeTreeQueue::initialize(const MergeTreeData::DataParts & parts)
{
    addVirtualParts(parts);
}
void ReplicatedMergeTreeQueue::addVirtualParts(const MergeTreeData::DataParts & parts)
{
    std::lock_guard lock(state_mutex);
    for (const auto & part : parts)
    {
        current_parts.add(part->name);
        virtual_parts.add(part->name);
    }
}
bool ReplicatedMergeTreeQueue::isVirtualPart(const MergeTreeData::DataPartPtr & data_part) const