Cache correctness investigations

[SgtCoDFish]

In #90 @inteon implemented some changes to help with cache correctness, since trust-manager has to work around some limitations currently present in controller-runtime (see kubernetes-sigs/controller-runtime#1962 which should help).

We believe that PR was an improvement, but there might be other issues that need addressing which Tim is looking into.

It seems like my PR just makes Secrets the same as the other resources, but there is a fix necessary to make everything work as intended (minimal caching etc.).
[and to avoid] duplicate caching (see this comment for more info: https://github.com/kubernetes-sigs/controller-runtime/blob/master/pkg/builder/options.go#L103-L132)
My current hypothesis is that we cache every resource 2 times (one time metadata only and one time fully), I still have to check by running an actual example.

This is a tracking issue for that work, which is likely a sensible prerequisite for a 0.4.0 release.

[inteon]

Here you can see that the "client" in bundle is also backed by a cache

So I think this comment is wrong:

So, when you make a "Get" request with that client, a new informer is started that will cache all resources of that type.

NOTE: Still not 100% certain