safekeeper: add separate tombstones map for deleted timelines

[jcsp]

Problem

Safekeepers left running for a long time use a lot of memory (up to the point of OOMing, on small nodes) for deleted timelines, because the Timeline struct is kept alive as a guard against recreating deleted timelines.

Closes: #6810

Summary of changes

• Create separate tombstones that just record a ttid and when the timeline was deleted.
• Add a periodic housekeeping task that cleans up tombstones older than a hardcoded TTL (24h)

I think this also makes #6766 un-needed, as the tombstone is also checked during deletion.

I considered making the overall timeline map use an enum type containing active or deleted, but having a separate map of tombstones avoids bloating that map, so that calls like get() can still go straight to a timeline without having to walk a hashmap that also contains tombstones.

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]

3024 tests run: 2909 passed, 0 failed, 115 skipped (full report)

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Flaky tests (2)

Postgres 14

• test_ondemand_wal_download_in_replication_slot_funcs: debug
• test_subscriber_restart: release

Code coverage* (full report)

• functions: 32.6% (6932 of 21274 functions)
• lines: 50.0% (54454 of 108936 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
93e85f7 at 2024-07-05T10:18:39.640Z :recycle:}

[arssher]

There are two related FIXMEs in timeline_global_map.rs, they also should be removed.