compute: Guarantee that subscribe response data is consolidated

[andrewrodriguez-m]

I thought this was guaranteed but couldn't find explicit documentation. Not having to handle unconsolidated data makes processing subscribe responses a little easier.

Motivation

This is already guaranteed here

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• This PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way) and therefore is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:

[teskje]

This seems reasonable. I've added a second commit that adds comments to the consolidation sites to point out that we do this now to fulfill a requirement of the compute protocol.