Fix DeltaLayer dumping

[arpad-m]

Problem

Before, DeltaLayer dumping (via cargo run --release -p pagectl -- print-layer-file ) would crash as one can't call Handle::block_on in an async executor thread.

Summary of changes

Avoid the problem by using DeltaLayerInner::load_keys to load the keys into RAM (which we already do during compaction), and then load the values one by one during dumping.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[arpad-m]

FTR, I confirmed locally that the test caused a crash before this patch.

[github-actions]

1624 tests run: 1549 passed, 0 failed, 75 skipped (full report)

---

_{The comment gets automatically updated with the latest test results
5f99267 at 2023-08-18T22:23:48.116Z :recycle:}