refactor(keystore): MlsPendingMessage has no real primary key

[coriolinus]

What's new in this PR

We'd been faking it with an inefficient workaround, but we now have the trait infrastructure to handle this properly.

---

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• The PR Title
  • conforms to the style of semantic commits messages¹ supported in Wire's Github Workflow²
  • contains a reference JIRA issue number like SQPIT-764
  • answers the question: If merged, this PR will: ... ³

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2. https://github.com/wireapp/.github#usage
3. E.g. feat(conversation-list): Sort conversations by most emojis in the title #SQPIT-764.

[github-actions]

Bencher Report

Branch	prgn/refactor/mlspendingmessage-has-no-primary-key
Testbed	ubuntu-latest

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Benchmark	Latency	microseconds (µs)
Commit add f(group size)/cs1/mem/1002	📈 view plot ⚠️ NO THRESHOLD	15,433.00 µs
Commit add f(group size)/cs1/mem/2	📈 view plot ⚠️ NO THRESHOLD	673.90 µs
Commit add f(group size)/cs1/mem/202	📈 view plot ⚠️ NO THRESHOLD	3,795.70 µs
Commit add f(group size)/cs1/mem/402	📈 view plot ⚠️ NO THRESHOLD	7,033.10 µs
Commit add f(group size)/cs1/mem/602	📈 view plot ⚠️ NO THRESHOLD	10,480.00 µs
Commit add f(group size)/cs1/mem/802	📈 view plot ⚠️ NO THRESHOLD	12,766.00 µs
Commit add f(number clients)/cs1/mem/1002	📈 view plot ⚠️ NO THRESHOLD	980,780.00 µs
Commit add f(number clients)/cs1/mem/2	📈 view plot ⚠️ NO THRESHOLD	654.48 µs
Commit add f(number clients)/cs1/mem/202	📈 view plot ⚠️ NO THRESHOLD	79,031.00 µs
Commit add f(number clients)/cs1/mem/402	📈 view plot ⚠️ NO THRESHOLD	213,970.00 µs
Commit add f(number clients)/cs1/mem/602	📈 view plot ⚠️ NO THRESHOLD	422,010.00 µs
Commit add f(number clients)/cs1/mem/802	📈 view plot ⚠️ NO THRESHOLD	669,090.00 µs
Commit pending proposals f(group size)/cs1/mem/1002	📈 view plot ⚠️ NO THRESHOLD	114,390.00 µs
Commit pending proposals f(group size)/cs1/mem/2	📈 view plot ⚠️ NO THRESHOLD	22,427.00 µs
Commit pending proposals f(group size)/cs1/mem/202	📈 view plot ⚠️ NO THRESHOLD	40,684.00 µs
Commit pending proposals f(group size)/cs1/mem/402	📈 view plot ⚠️ NO THRESHOLD	56,378.00 µs
Commit pending proposals f(group size)/cs1/mem/602	📈 view plot ⚠️ NO THRESHOLD	75,667.00 µs
Commit pending proposals f(group size)/cs1/mem/802	📈 view plot ⚠️ NO THRESHOLD	91,140.00 µs
Commit pending proposals f(pending size)/cs1/mem/1	📈 view plot ⚠️ NO THRESHOLD	15,988.00 µs
Commit pending proposals f(pending size)/cs1/mem/101	📈 view plot ⚠️ NO THRESHOLD	112,650.00 µs
Commit pending proposals f(pending size)/cs1/mem/21	📈 view plot ⚠️ NO THRESHOLD	32,310.00 µs
Commit pending proposals f(pending size)/cs1/mem/41	📈 view plot ⚠️ NO THRESHOLD	53,736.00 µs
Commit pending proposals f(pending size)/cs1/mem/61	📈 view plot ⚠️ NO THRESHOLD	72,500.00 µs
Commit pending proposals f(pending size)/cs1/mem/81	📈 view plot ⚠️ NO THRESHOLD	92,989.00 µs
Commit remove f(group size)/cs1/mem/1002	📈 view plot ⚠️ NO THRESHOLD	10,483.00 µs
Commit remove f(group size)/cs1/mem/2	📈 view plot ⚠️ NO THRESHOLD	517.66 µs
Commit remove f(group size)/cs1/mem/202	📈 view plot ⚠️ NO THRESHOLD	2,026.40 µs
Commit remove f(group size)/cs1/mem/402	📈 view plot ⚠️ NO THRESHOLD	3,565.00 µs
Commit remove f(group size)/cs1/mem/602	📈 view plot ⚠️ NO THRESHOLD	5,440.30 µs
Commit remove f(group size)/cs1/mem/802	📈 view plot ⚠️ NO THRESHOLD	7,432.80 µs
Commit remove f(number clients)/cs1/mem/1002	📈 view plot ⚠️ NO THRESHOLD	13,087.00 µs
Commit remove f(number clients)/cs1/mem/2	📈 view plot ⚠️ NO THRESHOLD	132,730.00 µs
Commit remove f(number clients)/cs1/mem/202	📈 view plot ⚠️ NO THRESHOLD	108,420.00 µs
Commit remove f(number clients)/cs1/mem/402	📈 view plot ⚠️ NO THRESHOLD	85,487.00 µs
Commit remove f(number clients)/cs1/mem/602	📈 view plot ⚠️ NO THRESHOLD	61,142.00 µs
Commit remove f(number clients)/cs1/mem/802	📈 view plot ⚠️ NO THRESHOLD	37,060.00 µs
Commit update f(group size)/cs1/mem/1002	📈 view plot ⚠️ NO THRESHOLD	132,030.00 µs
Commit update f(group size)/cs1/mem/2	📈 view plot ⚠️ NO THRESHOLD	676.92 µs
Commit update f(group size)/cs1/mem/202	📈 view plot ⚠️ NO THRESHOLD	27,320.00 µs
Commit update f(group size)/cs1/mem/402	📈 view plot ⚠️ NO THRESHOLD	53,645.00 µs
Commit update f(group size)/cs1/mem/602	📈 view plot ⚠️ NO THRESHOLD	81,311.00 µs
Commit update f(group size)/cs1/mem/802	📈 view plot ⚠️ NO THRESHOLD	106,490.00 µs

🐰 View full continuous benchmarking report in Bencher

[fewerner]

To me this looks fine, ty!