feat: support load dataset in parallel

[codecov[bot]]

Codecov Report

Patch coverage: 86.95% and project coverage change: +0.05% :tada:

Comparison is base (59ab0b4) 70.03% compared to head (ce8f9ee) 70.09%.

:exclamation: Current head ce8f9ee differs from pull request most recent head 714a368. Consider uploading reports for the commit 714a368 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #749 +/- ## ========================================== + Coverage 70.03% 70.09% +0.05% ========================================== Files 64 65 +1 Lines 12186 12207 +21 ========================================== + Hits 8535 8556 +21 Misses 2868 2868 Partials 783 783 ``` | [Files Changed](https://app.codecov.io/gh/gorse-io/gorse/pull/749?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gorse-io) | Coverage Δ | | |---|---|---| | [master/tasks.go](https://app.codecov.io/gh/gorse-io/gorse/pull/749?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gorse-io#diff-bWFzdGVyL3Rhc2tzLmdv) | `59.61% <75.00%> (+0.24%)` | :arrow_up: | | [base/parallel/future.go](https://app.codecov.io/gh/gorse-io/gorse/pull/749?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gorse-io#diff-YmFzZS9wYXJhbGxlbC9mdXR1cmUuZ28=) | `100.00% <100.00%> (ø)` | | | [model/ranking/data.go](https://app.codecov.io/gh/gorse-io/gorse/pull/749?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gorse-io#diff-bW9kZWwvcmFua2luZy9kYXRhLmdv) | `82.74% <100.00%> (+0.17%)` | :arrow_up: | | [storage/data/database.go](https://app.codecov.io/gh/gorse-io/gorse/pull/749?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gorse-io#diff-c3RvcmFnZS9kYXRhL2RhdGFiYXNlLmdv) | `74.35% <100.00%> (+0.67%)` | :arrow_up: |

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