[Bug] 边shard不均导致backup很慢

### Bug Type (问题类型)

performance (性能下降)

### 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: RocksDB x nodes, HDD or SSD
- OS: xx CPUs, xx G RAM, Ubuntu 2x.x / CentOS 7.x
- Data Size: xx vertices, xx edges

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

执行边的备份，发现非常慢（点备份很快），命令为：
`bin/hugegraph --url http://127.0.0.1:8080 --graph hugegraph backup -t edge --directory ./backup_test`

通过源码发现问题应该是边shard不均匀，导致大部分的线程执行完各自的小分片之后退出，最后只会有1～2个线程在执行较大的分片。虽然配置了多线程，但最后只有一个线程在执行备份，其他线程均退出。

也打印了线程池的活跃情况，一开始全部活跃，到最后只有2个活跃。（忽略线程数与备份生成的文件数不一致的情况，修改过线程数）

### Vertex/Edge example (问题点 / 边数据举例)

_No response_

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

_No response_

Start by running the reported `bin/hugegraph --url http://127.0.0.1:8080 --graph hugegraph backup -t edge --directory ./backup_test` command and compare edge and vertex backup behavior. Inspect the backup path and shard workload using the thread-activity evidence in the issue; done means edge backup no longer leaves most threads idle and completes faster.

Written by the indexing model from the issue text.