compute: consolidate AllowCompaction commands

[teskje]

Defer the transmission of AllowCompaction commands to periodic controller maintenance, to reduce their volume and remove some noise from the command stream.

This changes the behavior slightly in that AllowCompaction commands can now be delayed by up to 1s more than previously. I don't foresee this to be an issue.

Motivation

• This PR adds a feature that has not yet been specified.

Tips for reviewer

No strong feelings on whether we should merge this. On one hand, it's always good to try to be more efficient, and having a less noisy command stream makes it easier to identify anomalies. On the other hand I'm not aware of any issues caused by the current volume of compute commands and this change does add some additional complexity to the controller.

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.