[Bug] 使用postgre作为backend数据库创建了多个连接，且连接没有自动关闭

### 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: 1.5.0 (Apache Release Version)
- Backend: postgresql
- OS: Ubuntu
- Data Size: xx vertices, xx edges

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

`bin/start-hugegraph.sh`启动完hugegraph后， 数据库会出现很多连接，我这里是76个连接

这些连接没有被自动释放或关闭，只有我`bin/stop-hugegraph.sh`后才会关闭


### Vertex/Edge example (问题点 / 边数据举例)

```javascript

```

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

```javascript

```