pageserver: tighten up code around SLRU dir key handling

[jcsp]

Problem

Changes in #9786 were functionally complete but missed some edges that made testing less robust than it should have been:

• is_key_disposable didn't consider SLRU dir keys disposable
• Timeline init_empty was always creating SLRU dir keys on all shards

The result was that when we had a bug (#10080), it wasn't apparent in tests, because one would only encounter the issue if running on a long-lived timeline with enough compaction to drop the initially created empty SLRU dir keys, and some CLog truncation going on.

Closes: https://github.com/neondatabase/cloud/issues/21516

Summary of changes

• Update is_key_global and init_empty to handle SLRU dir keys properly -- the only functional impact is that we avoid writing some spurious keys in shards >0, but this makes testing much more robust.
• Make test_clog_truncate explicitly use a sharded tenant

The net result is that if one reverts #10080, then tests fail (i.e. this PR is a reproducer for the issue)

[github-actions]

6456 tests run: 6159 passed, 0 failed, 297 skipped (full report)

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Flaky tests (4)

Postgres 17

• test_pageserver_small_inmemory_layers[True]: debug-x86-64
• test_check_visibility_map: debug-x86-64

Postgres 15

• test_pgdata_import_smoke[None-1024-RelBlockSize.MULTIPLE_RELATION_SEGMENTS]: release-arm64

Postgres 14

• test_pull_timeline[True]: release-x86-64

Code coverage* (full report)

• functions: 31.4% (8334 of 26538 functions)
• lines: 47.7% (65626 of 137593 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
2624f2e at 2024-12-12T19:21:42.114Z :recycle:}