k8s: Upgrade StatefulSet

upgrade.md

1. modify your container so you can signal it somehow to disable writes
2. update the containers with that change
3. copy existing stateful set and give it a new name
4. create a new stateful set from that
5. for each replica
   1. exec into old container, stop writes
   2. exec into new container, stop writes
   3. copy over data
   4. enable writes in new container
6. delete old stateful set