add per-slot-uid to getHeader and getPayload

[metachris]

📝 Summary

Add a unique ID per slot to getHeader and getPayload.

Closes #492

---

✅ I have run these commands

• make lint
• make test-race
• go mod tidy

[codecov-commenter]

Codecov Report

Merging #498 (af573ad) into develop (54eeaa2) will decrease coverage by 0.14%.
The diff coverage is 65.80%.

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@@             Coverage Diff             @@
##           develop     #498      +/-   ##
===========================================
- Coverage    70.24%   70.10%   -0.14%     
===========================================
  Files            6        8       +2     
  Lines          541     1291     +750     
===========================================
+ Hits           380      905     +525     
- Misses         132      337     +205     
- Partials        29       49      +20     

Flag	Coverage Δ
unittests	70.10% <65.80%> (-0.14%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cli/main.go	0.00% <0.00%> (ø)
cli/types.go	0.00% <0.00%> (ø)
server/mock_types.go	100.00% <ø> (ø)
server/types.go	70.00% <70.00%> (ø)
server/service.go	77.34% <75.31%> (+5.32%)	⬆️
server/utils.go	82.58% <85.15%> (+15.01%)	⬆️
server/mock_relay.go	88.44% <90.81%> (+5.58%)	⬆️
server/relay_entry.go	90.00% <100.00%> (+8.75%)	⬆️

... and 1 file with indirect coverage changes

[ralexstokes]

generally looks good! I'd consider more precise keying that is more flexible and "harder to get wrong", esp during times where this feature is most needed

[metachris]

Merging now, since it was re-reviewed by @ralexstokes already, and I added a commit with only a minor header key change