Remove dependency on k8s.io code

[twpayne]

Firstly, this project is really interesting, thank you, and I believe that statedb will be extremely useful as the core data store for many applications outside Cilium and Kubernetes.

However, the current dependency graph is huge and includes k8s.io/api, k8s.io/apimachinery, and k8s.io/client-go. This makes the project significantly less appealing as few non-Kubernetes projects want to pull in large sections of the huge Kubernetes codebase as a dependency, even if most of it will be eventually pruned by the linker.

This is a draft PR, and probably should not be merged, to indicate the effort required to make this code more generic:

• The Kubernetes reflector is simply removed, as this is application-specific code and not relevant to statedb's core functionality. It can be moved to the application using statedb.
• The single type (a generic set) from k8s.io/apimachinery/pkg/util/sets is vendored, removing the dependency on k8s.io/apimachinery.

This is a draft PR designed to encourage discussion, not to be merged.

[joamaki]

Thanks! Agree on removing the k8s specific code so let's go for that. I'll move it into cilium/cilium or into a statedb utils repo.

I would not vendor sets.Set since we can instead just use map[T]bool or map[T]struct{} internally where needed. Also CollectSet now returns the wrong type and it's used in cilium/cilium in places that expect the sets.Set type. I think we can just drop CollectSet for now.

Could you refactor the remaining sets.Set uses to just use map[T]bool? It's mostly just test+example and NewTable so should be straightforward.

[twpayne]

Could you refactor the remaining sets.Set uses to just use map[T]bool? It's mostly just test+example and NewTable so should be straightforward.

Done.

[twpayne]

I couldn't resist adding an extra commit to use map[T]struct{} instead of map[T]bool for internal set types. As you know, this reduces memory usage at the cost of slightly different ergonomics. However, memory usage is unlikely to be a significant factor here (all sets are currently temporary, either within a function or within a test).