pageserver: make heatmap generation additive

[VladLazar]

Problem

Previously, when cutting over to cold secondary locations,
we would clobber the previous, good, heatmap with a cold one.
This is because heatmap generation used to include only resident layers.

Once this merges, we can add an endpoint which triggers full heatmap
hydration on attached locations to heal cold migrations.

Summary of changes

With this patch, heatmap generation becomes additive. If we have a
heatmap from when this location was secondary, the new uploaded heatmap
will be the result of a reconciliation between the old one and the on
disk resident layers.

More concretely, when we have the previous heatmap:

1. Filter the previous heatmap and keep layers that are (a) present
   in the current layer map, (b) visible, (c) not resident. Call this set
   of layers visible_non_resident.
2. From the layer map, select all layers that are resident and visible.
   Call this set of layers resident.
3. The new heatmap is the result of merging the two disjoint sets.

Related #10541

[github-actions]

6750 tests run: 6434 passed, 0 failed, 316 skipped (full report)

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Flaky tests (5)

Postgres 17

• test_pgdata_import_smoke[8-1024-RelBlockSize.MULTIPLE_RELATION_SEGMENTS]: release-arm64-with-lfc, release-arm64-without-lfc
• test_pgdata_import_smoke[None-1024-RelBlockSize.MULTIPLE_RELATION_SEGMENTS]: release-arm64-with-lfc, release-arm64-without-lfc

Postgres 16

• test_tenant_import[None-local_fs]: release-arm64-with-lfc

Test coverage report is not available

_{The comment gets automatically updated with the latest test results
82b550a at 2025-01-30T21:18:32.791Z :recycle:}