Refactor DatasetResource and DatasetAccessResource

[bobbai00]

This PR refactors the codes style of the DatasetResource and DatasetAccessResource.

Specifically, the refactoring principles are:

1. Move all the function definition in object, that are only being used within the class, to the class
2. Uniform the way of interacting with jooq: use either jooq raw query, or Dao, not both.
3. Flatten single-kv-pair case classes.
4. Flatten the single-line function.

[aglinxinyuan]

LGTM! We had a offline discussion regarding the changes.