fix(pageserver): skip metadata compaction if LSN is not accumulated enough

[skyzh]

Problem

close #7937

Summary of changes

Only trigger metadata image layer creation if enough delta layers are accumulated.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

3204 tests run: 3065 passed, 0 failed, 139 skipped (full report)

---

Flaky tests (3)

Postgres 16

• test_pageserver_restarts_under_worload: release

Postgres 15

• test_pageserver_restarts_under_worload: release

Postgres 14

• test_sharding_split_compaction[compact-shard-ancestors-localonly]: debug

Code coverage* (full report)

• functions: 31.6% (6600 of 20917 functions)
• lines: 48.5% (51061 of 105214 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
d3e4f82 at 2024-06-06T16:04:28.317Z :recycle:}

[skyzh]

ready for review again :)