perf(PostCategory/PostTag): add binary relation index for performance

[D-Sketon]

What does it do?

#5604
To improve performance, I introduced indexes for binary relations to improve lookup speeds.

The overhead from find and findOne is now largely invisible on the flamegraph:

Screenshots

8x many-posts
before

after

1x many-posts
before

after

This is a huge boost for blogs with a super large number of posts, but hopefully this won't have any side effects on the generation of the site.

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[github-actions]

How to test

git clone -b perf/binary-relation-index https://github.com/D-Sketon/hexo.git
cd hexo
npm install
npm test

[coveralls]

Pull Request Test Coverage Report for Build 13067175174

Details

• 154 of 159 (96.86%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 99.448%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/models/binary_relation_index.ts	95	100	95.0%

Totals
Change from base Build 12902994421:	-0.04%
Covered Lines:	9728
Relevant Lines:	9782

---

💛 - Coveralls

[D-Sketon]

No hook will be triggered during the warehouse import process, so we must manually trigger the saveHook after loading the db.json.