InMemoryLayer: move end_lsn out of the lock

[arpad-m]

Problem

In some places, the lock on InMemoryLayerInner is only created to obtain end_lsn. This is not needed however, if we move end_lsn to InMemoryLayer instead.

Summary of changes

Make end_lsn a member of InMemoryLayer, and do less locking of InMemoryLayerInner. end_lsn is changed from Option<Lsn> into an OnceLock<Lsn>. Thanks to this change, we don't need to lock any more in three functions.

Part of #4743 . Suggested in #4905 (comment) .

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

[github-actions]

1584 tests run: 1509 passed, 0 failed, 75 skipped (full report)

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_{The comment gets automatically updated with the latest test results
9d91ddc at 2023-08-11T15:22:40.086Z :recycle:}

[arpad-m]

I've pushed a new version now that uses OneLock, and addressed all the review comments.