Automatic merge from submit-queue (batch tested with PRs 66152, 66406, 66218, 66278, 65660). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Refactor kubelet node status setters, add test coverage

This internal refactor moves the node status setters to a new package, explicitly injects dependencies to facilitate unit testing, and adds individual unit tests for the setters.

I gave each setter a distinct commit to facilitate review.

Non-goals:
- I intentionally excluded the class of setters that return a "modified" boolean, as I want to think more carefully about how to cleanly handle the behavior, and this PR is already rather large.
- I would like to clean up the status update control loops as well, but that belongs in a separate PR.

```release-note
NONE
```