Load payments in parallel using std::thread::scope

[martinsaposnic]

This significantly improves startup time for network-backed stores like VSS, where sequential reads incur per-key network latency.

[ldk-reviews-bot]

👋 Hi! I see this is a draft PR.
I'll wait to assign reviewers until you mark it as ready for review.
Just convert it out of draft status when you're ready for review!

[tnull]

As mentioned elsewhere, we should probably not do this with threads, as we aim to make more of node building fully async over time. I now opened #739 as an async-friendly alternative.

[tnull]

Closing this as superseded by #739.