Set result.not_modified_since as minimum of last_written_lsn and replay lsn

[knizhnik]

Problem

See https://neondb.slack.com/archives/C04DGM6SMTM/p1715785588061879?thread_ts=1715772441.057759&cid=C04DGM6SMTM

lastReplayedEndRecPtr is advanced after applying WAL record while last written LSN can be advanced while
applying WAL record this is why last_written_lsn can be larger than GetXLogReplayRecPtr(NULL).

Summary of changes

result.not_modified_since = Min(last_written_lsn, result.request_lsn);

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]

3078 tests run: 2951 passed, 0 failed, 127 skipped (full report)

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Flaky tests (2)

Postgres 15

• test_vm_bit_clear_on_heap_lock: debug

Postgres 14

• test_synthetic_size_while_deleting: release

Code coverage* (full report)

• functions: 31.3% (6382 of 20401 functions)
• lines: 47.7% (48630 of 101897 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
0a0e834 at 2024-05-18T12:46:16.291Z :recycle:}

[hlinnaka]

With this, you might read an old version of the page, no?

[hlinnaka]

With the repro at PR #7795, this panics:

PG:2024-05-17 16:04:26.250 GMT [1353937] PANIC:  neon_rm_redo: invalid lp
PG:2024-05-17 16:04:26.250 GMT [1353937] CONTEXT:  WAL redo at 0/15B3098 for neon/HOT_UPDATE: old_xmax: 4893, old_off: 102, old_infobits: [], flags: 0x10, new_xmax: 0, new_off: 103; blkref #0: rel 1663/5/16393, blk 0
PG:2024-05-17 16:04:26.252 GMT [1353932] LOG:  startup process (PID 1353937) was terminated by signal 6: Aborted

[hlinnaka]

This was fixed by PR #7795