Turn BlockLease associated type into an enum

[arpad-m]

Problem

The BlockReader trait is not ready to be asyncified, as associated types are not supported by asyncification strategies like via the async_trait macro, or via adopting enums.

Summary of changes

Remove the BlockLease associated type from the BlockReader trait and turn it into an enum instead, bearing the same name. The enum has two variants, one of which is gated by #[cfg(test)]. Therefore, outside of test settings, the enum has zero overhead over just having the PageReadGuard. Using the enum allows us to impl BlockReader without needing the page cache.

Part of #4743

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.
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• 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

[github-actions]

1588 tests run: 1513 passed, 0 failed, 75 skipped (full report)

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_{The comment gets automatically updated with the latest test results
b1f1c10 at 2023-08-14T11:49:51.881Z :recycle:}

[LizardWizzard]

Looks good to me