## Problem
When a secondary location is trying to catch up while a tenant is
receiving new writes, it can become quite wasteful:
- Downloading L0s which are soon destroyed by compaction to L1s
- Downloading older layer files which are soon made irrelevant when
covered by image layers.
## Summary of changes
Sort the layer files in the heatmap:
- L0 layers are the lowest priority
- Other layers are sorted to download the highest LSNs first.
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