Add RecordQueryConcatenationPlan for non-overlapping unions

[nschiefer]

When implementing a union that's guaranteed to be non-overlapping, the planner could create a concatenation plan that would execute more efficiently.

[alecgrieser]

Is the idea that this wouldn't be order preserving? Or it would only order preserving if you knew that all of plan a's ordering keys were less than plan b's ordering keys, so you could safely concatenate plan a and then plan b?

[nschiefer]

I was imagining the latter. It's a bit niche, but there was a motivating use case.