[Bug] SingleNode模式下IndexBuilder 无法build

### Bug Type (问题类型)

others (please edit later)

### Before submit

- [X] 我已经确认现有的 [Issues](https://github.com/apache/hugegraph/issues) 与 [FAQ](https://hugegraph.apache.org/docs/guides/faq/) 中没有相同 / 重复问题 (I have confirmed and searched that there are no similar problems in the historical issue and documents)

### Environment (环境信息)

- Server Version: 1.0.0 (Apache Release Version)
- Backend: Mysql
- OS: xx CPUs, xx G RAM, Ubuntu 2x.x / CentOS 7.x

this.submitTask(task);

### Expected & Actual behavior (期望与实际表现)

schema.indexLabel("personByAge")
.onV("person")
.by("age")
.range()
.ifNotExist()
.create();

这个asyncwait无法完成

https://github.com/apache/incubator-hugegraph/blob/6c0d596eac57d5bfc80cff86cb5565818d876817/hugegraph-core/src/main/java/org/apache/hugegraph/task/StandardTaskScheduler.java#L222

这里不是return this.submitTask(task);
scheduler的tasks里面没有这个task

### Vertex/Edge example (问题点 / 边数据举例)

```javascript
无
```

### Schema [VertexLabel, EdgeLabel, IndexLabel] (元数据结构)

```javascript
loader的test中 testVertexPrimaryKeyUnfold 和 testVertexPrimaryKeyUnfoldWithMapping 这两个case
```