pageserver: only store SLRUs on shard zero

[jcsp]

Problem

Since #9423 the non-zero shards no longer need SLRU content in order to do GC. This data is now redundant on shards >0.

Closes: #7512

Summary of changes

• Avoid storing SLRUs on non-zero shards
• Avoid storing checkpoint on non-zero shards

[github-actions]

5499 tests run: 5255 passed, 1 failed, 243 skipped (full report)

---

Failures on Postgres 17

• test_timeline_retain_lsn[offload-parent-False]: debug-x86-64

# Run all failed tests locally:
scripts/pytest -vv -n $(nproc) -k "test_timeline_retain_lsn[debug-pg17-offload-parent-False]"

Flaky tests (1)

Postgres 16

• test_timeline_archival_chaos: release-arm64

Test coverage report is not available

_{The comment gets automatically updated with the latest test results
001341a at 2024-11-18T11:18:39.689Z :recycle:}