Only decode rows up to demand

[antiguru]

Decode rows in rendering up to a demand limit.

Motivation

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[antiguru]

Alright, this doesn't work as-is. In a lot of places, we evaluate an mfp, which assumes input_arity columns, and this change modifies the input_arity, without actually updating it.

[ggevay]

Converting to draft, as there are some test failures, and @antiguru said he is not planning to work on this in the near future. (I might pick it up if I have time.)