remove last applied replicas when statefulset is changed

[yuchen-db]

add a logic in rollout operator to remove last applied replicas when statefulset is changed. This should prevent any surprise when we omit replicas value from manifest and trigger a client-side apply

[jnyi]

will it be added back causing race condition? would you mind testing:

1. only deploy rollout-operator
2. check the specific key is removed
3. kubecfg apply ONLY the db statefulset
4. the keys are not added back

[yuchen-db]

will it be added back causing race condition? would you mind testing:

1. only deploy rollout-operator
2. check the specific key is removed
3. kubecfg apply ONLY the db statefulset
4. the keys are not added back

two cases for step 3:
a. replicas field is omitted, then the keys will not be added back
b. replicas field exists, then the keys will be added back, but will be removed shortly at next reconcile