Use OnceCell instead of SetLock

[Kitt3120]

Rust supports lazy loading since 1.70 with OnceCell and OnceLock, and especially since 1.80 with LazyCell and LazyLock. There is no need for SetLock to exist.

1. Remove SetLock
2. Refactor code to use OnceCell instead

[Kitt3120]

Seems like OnceLock is the one we actually want. Keeping the branch and issue name, don't be confused. :)