Fix IN and NOT IN queries

[kasparsd]

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Fixes #977.

• [x] Fix the core issue with preparing multiple values for IN and NOT IN queries.
• [x] Add unit tests for the new helper methods and the core DB query logic. Finished and confirmed by @kidunot89 :+1:

[lkraav]

Sup @kasparsd anything I can do to help here? Where did it get stuck?

[kasparsd]

@lkraav I started working on this before we had the dev environment bundled with the plugin which can run unit tests, so the remaining job is to add the unit tests and do some smoke testing with some actual data.

[kidunot89]

@kasparsd All ready for your review.