refactor(post): async/await

[curbengh]

What does it do?

#3328

Supersedes #3939

How to test

git clone -b post-async https://github.com/curbengh/hexo.git
cd hexo
npm install
npm test

Screenshots

Pull request tasks

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

[curbengh]

This is potentially a performance sensitive part. As bluebird sometimes can be significantly faster than async/await, need to verify performance.

[coveralls]

Coverage decreased (-0.0008%) to 97.085% when pulling 5d10f7f on curbengh:post-async into 663f1eb on hexojs:master.

[curbengh]

#3328 (comment)