storage: introduce search interface with scoring and filtering

[roidelapluie]

Which issue(s) does the PR fix:

Release notes for end users (ALL commits must be considered).

Reviewers should verify clarity and quality.

NONE

[aknuds1]

Directionally good, but my review uncovered some issues and refactoring opportunities.

[aknuds1]

Having looked it over again, I'd mostly like to suggest making storage.mergeSearchSets stream results when hints.CompareFunc is nil. This is to avoid full materialization when possible, with high-cardinality performance in mind.

Please take a look at my suggestions :)

Not requesting changes any longer, but not done reviewing either :)

[aknuds1]

In addition to the comments, I think that noopQuerier should also be made to implement Searcher for completeness:

// Compile-time assertion that noopQuerier implements Searcher.
var _ Searcher = noopQuerier{}

Otherwise, I think it looks good after an in-depth review.

[aknuds1]

Mostly LGTM, but there's a new change in this revision that seems highly unusual to me, i.e. to return nil after the first Warnings() call to iterators.

[aknuds1]

LGTM, thanks for applying my feedback!