Safekeeper peer recovery preparatory patches

[arssher]

Implements #4875

[arssher]

@petuhovskiy Trying to make this easier to digest I attempt to do it in several smaller commits -- the ones pushed here are ready for review.

[github-actions]

1624 tests run: 1550 passed, 0 failed, 74 skipped (full report)

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Flaky tests (1)

Postgres 14

• test_crafted_wal_end[last_wal_record_xlog_switch_ends_on_page_boundary]: release

_{The comment gets automatically updated with the latest test results
8f0ae23 at 2023-08-29T19:38:46.837Z :recycle:}

[knizhnik]

I wonder if this recovery can be somehow combined with retrieving at compute WAL needed for logical replication? In principle - the approach is similar: we need to send WAL till some boundary (in this case determined by logical replication slot) to walproposer.

Or it is better not to mix this two things?

[arssher]

Or it is better not to mix this two things?

These are not very related. Interface for fetching WAL from safekeepers by pg protocol exists for a long time, and can be used for logical replication as well, in fact we already have fetching code in walproposer (which I plan to remove soon, but anyway, it is trivial). This patchset extends so that not committed part can also be dynamically fetched, but that's not much needed for replication, as not committed part most often can be still on the compute as it generates it.

I still think that hardest part about logical repl is persistency of replication slots and historical snapshots...

[petuhovskiy]

I forgot that this PR is splitted into commits and reviewed all changed as usual (altogether in Files changed tab).

Overall LGTM, let's merge and deploy.