block cancellations: after validation, check if request is latest

[metachris]

📝 Summary

An earlier request could override a newer request if the validation was slow.

This PR adds a receivedAt check, to check after validation whether the request is still the latest.

---

✅ I have run these commands

• make lint
• make test-race
• go mod tidy
• I have seen and agree to CONTRIBUTING.md

[codecov-commenter]

Codecov Report

Merging #214 (85d663d) into main (285eff3) will decrease coverage by 0.00%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##             main     #214      +/-   ##
==========================================
- Coverage   23.94%   23.93%   -0.01%     
==========================================
  Files          19       19              
  Lines        2339     2365      +26     
==========================================
+ Hits          560      566       +6     
- Misses       1701     1719      +18     
- Partials       78       80       +2     

Flag	Coverage Δ
unittests	23.93% <25.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
datastore/datastore.go	33.33% <ø> (+0.83%)	⬆️
services/api/service.go	17.13% <8.33%> (-0.17%)	⬇️
datastore/redis.go	52.28% <35.00%> (-1.78%)	⬇️

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