feat: allow detaching from ancestor for timelines without writes

[koivunej]

The first implementation #7456 did not include index_part.json changes in an attempt to keep amount of changes down. Tracks the historic reparentings and earlier detach in index_part.json.

• index_part.json receives a new field lineage: Lineage
• Lineage is queried through RemoteTimelineClient during basebackup, creating PREV LSN: none for the invalid prev record lsn just as it would had been created for a newly created timeline
• as struct IndexPart grew, it is now boxed in places

Cc: #6994

[github-actions]

3060 tests run: 2927 passed, 0 failed, 133 skipped (full report)

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Flaky tests (3)

Postgres 15

• test_gc_aggressive: debug
• test_vm_bit_clear_on_heap_lock: debug

Postgres 14

• test_gc_aggressive: debug

Code coverage* (full report)

• functions: 31.4% (6329 of 20161 functions)
• lines: 47.3% (47710 of 100932 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
f2853dc at 2024-05-10T18:13:33.945Z :recycle:}

[problame]

Please create a PR against the RFC PR with a section on Lineage. Or, work with John to add it.

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This one needs updating:

neon/pageserver/src/tenant/timeline.rs

Line 4354 in 5caaf2b

/// - has prev_lsn in remote storage (temporary restriction)

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Didn't have time to review the test and won't have because I'm off tomorrow and Friday.

[koivunej]

Please create a PR against the RFC PR with a section on Lineage. Or, work with John to add it.

I've already added it, it's for WAL DR (I've pushed comments around the "why").

[problame]

I've already added it,

Where?

[koivunej]

I've already added it,

Where?

https://github.com/neondatabase/neon/blob/d56679ee262e8bf85db2a24488922c33b3db975b/docs/rfcs/033-ancestor-deletion.md#wal-based-disaster-recovery

[problame]

Ok. I think it would be useful to somehow link the Rust struct and that section of the RFC.
It wasn't clear to me while reviewing that we need the lineage for WAL-based DR.
When I reviewed, it just read to me like "we're doing some book-keeping to make future debugging / troubleshooting easier".
I don't think there's any mentioning in the struct Lineage doc comment.

[koivunej]

Adding that to the TODO.