feat(rtc): maintain dirty and uploaded IndexPart

[koivunej]

RemoteTimelineClient maintains a copy of "next IndexPart" as a number of fields which are like an IndexPart but this is not immediately obvious. Instead of multiple fields, maintain a dirty ("next IndexPart") and clean ("uploaded IndexPart") fields.

Additional cleanup:

• rename IndexPart::disk_consistent_lsn accessor duplicated_disk_consistent_lsn
  • no one except scrubber should be looking at it, even scrubber is a stretch
  • remove usage elsewhere (pagectl used by tests, metadata scan endpoint)
• serialize index part before the index upload operation
  • avoid upload operation being retried because of serialization error
  • serialization error is fatal anyway for timeline -- it can only make transient local progress after that, at least the error is bubbled up now
• gather exploded IndexPart fields into single actual UploadQueueInitialized::dirty of which the uploaded snapshot is serialized
• implement the long wished monotonicity check with the clean IndexPart with an assertion which is not expected to fire

Continued work from #7860 towards next step of #6994.

[github-actions]

3198 tests run: 3059 passed, 0 failed, 139 skipped (full report)

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Flaky tests (1)

Postgres 14

• test_hot_standby_gc[True]: debug

Code coverage* (full report)

• functions: 31.4% (6563 of 20873 functions)
• lines: 48.4% (50784 of 104942 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
a2463ca at 2024-06-04T11:50:59.590Z :recycle:}

[koivunej]

Perhaps cloning this to a clean on completed upload isn't a problem, I was remembering we would use String in IndexPart::layer_metadata: HashMap<_, LayerFileMetadata> but that is the fixed length LayerName.

[arssher]

Don't have a lot of context here so someone else should have a look, but looks reasonable.