* image, store/tooling: fix how validation-sets and required revision was set for snaps that were controlled by validation-sets. Mirror how it's done for the snap validate --refresh.
* image: fix unit tests and optimize a bit how many times we call validationSetsAndRevisionForSnap
* image: wrong revision in comment
* image: review feedback, add TODO around missing logic for optional snaps, and add unit test for conflict logic that is now added
* image: review feedback, be a bit more consistent in naming between keys and actual validation-set objects