safekeeper: use WAL_SEGMENT_SIZE for empty timeline state

[erikgrinaker]

Problem

TimelinePersistentState::empty(), used for tests and benchmarks, had a hardcoded 16 MB WAL segment size. This caused confusion when attempting to change the global segment size.

Summary of changes

Inherit from WAL_SEGMENT_SIZE in TimelinePersistentState::empty().

[github-actions]

5382 tests run: 5162 passed, 0 failed, 220 skipped (full report)

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Flaky tests (2)

Postgres 17

• test_replica_query_race: release-x86-64
• test_create_churn_during_restart: debug-x86-64

Code coverage* (full report)

• functions: 31.8% (7888 of 24834 functions)
• lines: 49.5% (62456 of 126258 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
7f352d9 at 2024-11-12T18:55:48.972Z :recycle:}