Revisit node shutdown clean up and improve testing

[pebrc]

#5629 implemented a minimal fix to address the issue with auto_expand_replicas and node shutdown.

We should look into improving the testing around that e.g through a continuous check during mutation tests that ensure that at most as many shutdowns exist as the change budget allows.

We could also improve the code a bit and for example move all clean up to the top of the upgrade function.