catalog: simplify open_debug_catalog implementation

[benesch]

Ignore for now. This failed CI when it was in a slightly different form (using NOW_ZERO instead of SYSTEM TIME). I'm hoping this form works better.

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All of these tests are fine with using SYSTEM_TIME as the NowFn.

Motivation

• This PR refactors existing code.

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

[jkosh44]

@benesch Give me a ping if/when this is ready for review.

[benesch]

It's ready! CI seems happy with this.

[jkosh44]

LGTM

[benesch]

Thank you!