Testing improvements for PruneAcknowledgements rpc

[damiannolan]

Summary

Testing improvements to PruneAcknowledgements suggested by @colin-axner in #5444

• Add a test case where a middle sequence doesn't have an ack stored (because a timeout occurred) Add pruning logic, implementation in message server. #5444 (comment)
• Add a test where limit is set higher (attempting to prune post upgrade packet sends) demonstrating that this is safe, and only packet sequences within range are deleted. Add pruning logic, implementation in message server. #5444 (comment)
• nit: Add pruning logic, implementation in message server. #5444 (comment)

---

For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged/assigned

[ThanhNhann]

Can I handle this issue?

[DimitrisJim]

Absolutely @ThanhNhann, have at it!