pageserver/config: disable image layer creation check threshold

[VladLazar]

Problem

Christian diagnosed the compaction change in #7230 as the root cause for slow find_lsn_for_timestamp in prod.

The essence of the problem is that Timeline::last_image_layer_creation_check_at is updated after the first partition, causing all remaining ones to skip image layer creation.

Details: https://neondb.slack.com/archives/C06UEMLK7FE/p1713380092262789

Summary of changes

As a quick fix, we effectively disable this check while we figure out how to inhibit image layer creation check
more gracefully. If it turns out that the solution shouldn't be wal based, we'll gracefully deprecate the config.

[problame]

(Edited the PR description)

[github-actions]

2760 tests run: 2641 passed, 1 failed, 118 skipped (full report)

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Failures on Postgres 15

• test_auth_failures[True]: debug

# Run all failed tests locally:
scripts/pytest -vv -n $(nproc) -k "test_auth_failures[debug-pg15-True]"

Test coverage report is not available

_{The comment gets automatically updated with the latest test results
bea69ef at 2024-04-17T20:30:34.887Z :recycle:}

[VladLazar]

Should probably close this in favour of https://github.com/neondatabase/aws/pull/1270 and implement a proper fix.

[VladLazar]

Closing in favour of proper fix: #7420