LFC prewarming with only light locking

[MMeent]

Problem

We want to prewarm the LFC, preferably without blocking any other processes (or at least blocking the least amount of processes

Summary of changes

See file_cache_internal.h for implementation details.

[github-actions]

7304 tests run: 6936 passed, 4 failed, 364 skipped (full report)

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Failures on Postgres 15

• test_lfc_prewarm: release-x86-64, release-arm64

Failures on Postgres 14

• test_lfc_prewarm: release-x86-64, release-arm64

# Run all failed tests locally:
scripts/pytest -vv -n $(nproc) -k "test_lfc_prewarm[release-pg14] or test_lfc_prewarm[release-pg14] or test_lfc_prewarm[release-pg15] or test_lfc_prewarm[release-pg15]"

Flaky tests (3)

Postgres 16

• test_lfc_prewarm: release-arm64
• test_pageserver_compaction_detach_ancestor_smoke: release-arm64

Postgres 14

• test_sharding_smoke: release-arm64

Test coverage report is not available

_{The comment gets automatically updated with the latest test results
0f93d01 at 2025-01-09T22:46:06.378Z :recycle:}