pagebench: fixup after is_rel_block_key changes in #6266

[problame]

PR #6266 broke the getpage_latest_lsn benchmark.

Before this patch, we'd fail with

not implemented: split up range

because r.start = rel size key and r.end = rel size key + 1.

The filtering of the key ranges in that loop is a bit ugly, but,
I measured:

• setup with 180k layer files (20k tenants * 9 layers).
• total physical size is 463GiB
• 5k tenants, the range filtering takes 0.6 seconds on an i3en.3xlarge.
  That's a tiny fraction of the overall time it takes for pagebench to get ready to send requests. So, this is good enough for now / there are other bottlenecks that are bigger.

[github-actions]

2238 tests run: 2154 passed, 0 failed, 84 skipped (full report)

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Flaky tests (2)

Postgres 16

• test_pageserver_init_node_id: debug

Postgres 15

• test_crafted_wal_end[last_wal_record_crossing_segment]: debug

Code coverage (full report)

• functions: 54.8% (10145 of 18521 functions)
• lines: 81.6% (58142 of 71265 lines)

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_{The comment gets automatically updated with the latest test results
51666d1 at 2024-01-08T20:10:12.819Z :recycle:}