The "Saturate" phase of StatefulSet e2e tests verifies orderly startup
by controlling when each Pod is allowed to report Ready.
If a Pod unexepectedly goes down during the test, the replacement Pod
created by the controller will forget if it was already allowed to
report Ready.
After this change, the signal that allows each Pod to report Ready is
persisted in the Pod's PVC. Thus, the replacement Pod will remember that
it was already told to proceed to a Ready state.
