Register custom xlog reader callbacks for on-demand WAL download in StartupDecodingContext

[knizhnik]

Problem

See #8931
On-demand WAL download are not set in all cases where WAL is accessed by logical replication

Summary of changes

Set customer xlog reader handles in StartupDecodingContext

Related changes in Postgres modules:

neondatabase/postgres#495
neondatabase/postgres#496
neondatabase/postgres#497
neondatabase/postgres#498

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

[orca-security-us]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

[github-actions]

4994 tests run: 4830 passed, 0 failed, 164 skipped (full report)

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Flaky tests (8)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64
• test_obsolete_slot_drop[cross-validation]: release-arm64
• test_pg_regress[4]: debug-x86-64
• test_scrubber_physical_gc[4]: debug-x86-64, release-arm64
• test_tenant_config: debug-x86-64

Postgres 16

• test_lr_with_slow_safekeeper: release-arm64

Postgres 15

• test_scrubber_physical_gc[4]: release-arm64

Code coverage* (full report)

• functions: 31.8% (7415 of 23294 functions)
• lines: 49.9% (59571 of 119380 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
f3d520e at 2024-09-16T18:52:49.065Z :recycle:}

[hlinnaka]

Some of the callers of CreateDecodingContext and CreateInitDecodingContext pass read_local_xlog_page, and some pass logical_read_xlog_page. Does NeonOnDemandXLogReaderRoutines work as a replacement for both?

CreateReplicationSlot() passes .segment_open = WalSndSegmentOpen. WalSndSegmentOpen has some code to deal with historic (postgres) timelines. I'm not sure if that code is reachable on Neon, but this makes us skip it.

I think I would be more comfortable if NeonOnDemandXLogReaderRoutines() was just a wrapper around the original routines, so that the original routines would still actually do the opening and reading of the file, after NeonOnDemandXLogReaderRoutines() has performed the download.

Needs tests. In particular, tests for the cases where we were not doing on-demand download without this PR.