feat: copy delta layer prefix or "truncate"

[koivunej]

Problem

For "timeline ancestor merge" or "timeline detach," we need to "cut" delta layers at particular LSN. The name "truncate" is not used as it would imply that a layer file changes, instead of what happens: we copy keys with Lsn less than a "cut point".

Cc: #6994

Summary of changes

Add the "copy delta layer prefix" operation to DeltaLayerInner, re-using some of the vectored read internals. The code is cfg(test) until it will be used later with a more complete integration test.

[github-actions]

2760 tests run: 2642 passed, 0 failed, 118 skipped (full report)

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Code coverage* (full report)

• functions: 28.1% (6475 of 23065 functions)
• lines: 46.8% (45603 of 97424 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
377f5ca at 2024-04-17T09:38:59.817Z :recycle:}