Stateful Sets

Initially called Pet Sets in v1.3, changed to StatefulSets and made stable in v1.9
Pod name has postfix numbers (an index) to have deterministic pods with state and volumes attached to it will also be deterministic.
- index start with 0 to n-1
- Also allows DNS resolution deterministic as other services can find the pod by the deterministic pod name.
- scaling goes from 0 to n-1
- teardown or scaledown will start from n-1 to 0