pageserver: fix unlogged relations with sharding

[jcsp]

Problem

• pageserver: unlogged relations (INIT_FORKNUM) not handled properly on sharded tenants #7451

INIT_FORKNUM blocks must be stored on shard 0 to enable including them in basebackup.

This issue can be missed in simple tests because creating an unlogged table isn't sufficient -- to repro I had to create an index on an unlogged table (then restart the endpoint).

Closes: #7451

Summary of changes

• Add a reproducer for the issue.
• Tweak the condition for key_is_shard0 to include anything that isn't a normal relation block and any normal relation block whose forknum is INIT_FORKNUM.
• To enable existing databases to recover from the issue, add a special case that omits relations if they were stored on the wrong INITFORK. This enables postgres to start and the user to drop the table and recreate it.

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]

2772 tests run: 2654 passed, 0 failed, 118 skipped (full report)

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Flaky tests (1)

Postgres 15

• test_vm_bit_clear_on_heap_lock: debug

Code coverage* (full report)

• functions: 28.1% (6460 of 23024 functions)
• lines: 46.8% (45586 of 97442 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
e55cdbb at 2024-04-22T11:54:47.808Z :recycle:}

[jcsp]

Pushed a change to how the workaround for existing broken tenants works: we can't directly know whether a given relation is problematic, we have to try reading it and ignore the error if it's a key that could have this issue.

This works with the real life example data we have.