Avoid int overflows for agglomerate tree id

[fm3]

Protobuf skeletons only support int tree ids, not long. We math.abs the id now as a hash function so that a unique deterministic tree color can be selected but no overflows occur, as the frontend won’t handle negative tree ids.

Steps to test:

• load agglomerate skeletons for agglomerates with very large ids
• they should show

Issues:

• fixes Agglomerate Skeleton request returned negative tree ids #6467

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• added changelog entry
• Needs datastore update after deployment
• Ready for review

[philippotto]

Great 👍