persist: split out a maelstrom-persist-base mzimage

[danhhz]

And then MZFROM it in maelstrom-persist. This way we don't have to re-install the apt things or maelstrom every time we get a CI builder with a cold cache.

Motivation

• This PR refactors existing code.

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.
• 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).
• This PR includes the following user-facing behavior changes:

[bkirwi]

On board in principle... I'll check back in once the build is green!

[danhhz]

The build failure is that @benesch has to create the new image in dockerhub :D

[benesch]

Should be all good with Docker Hub now.

[danhhz]

TFTRs!