k8swatch: convert PodWatcher to reconciler

[milas]

PodWatcher is now kubernetesdiscovery.Reconciler. Almost all
the logic is the same, though some stuff moved around. Most notable
change is that the check to decide whether to skip a status update
now happens against the API object instead of the store object.

[milas]

There's a few more test cases from pod_watcher_test.go that vanished here that I want to re-introduce. Some of them (e.g. making sure we don't do bad ResourceVersion string comparisons) should be reconciler tests; some of them are more about manifest behavior and need to get covered by the k8swatch.ManifestSubscriber tests (I think they all are, but made a list and need to compare to ensure match-y match-y behavior.) There might be a couple that become "upper" tests because they are exercising some weird inter-play between manifest-driven specs + actual watch behavior.

FYI There are already (many) "integration" tests in upper_test.go which are currently passing (🎉) using the k8swatch.ManifestSubscriber store subscriber + kubernetesdiscovery.Reconciler apiserver controller.

[milas]

This also fixes #4557. When there is no controller registered for a type, the controller-runtime client caching does not behave as expected, and will consistently return NotFound for some entities. When this happens, even adjusting the resync period to be very short doesn't cause it to become consistent.

We need to dive into the client caching behavior more, but this at least ensures things don't behave erratically.