When a changeset is updated but remains an auto-draft, ensure the post_date
for the auto-draft posts remains the same so that it will be
garbage-collected at the same time by
if the changeset is updated to be a draft then update the posts
to have a far-future post_date so that they will never be garbage collected
unless the changeset post itself is deleted.
When a changeset is updated to be a persistent draft or to be scheduled for publishing, then transition any dependent auto-drafts to a draft status so that they likewise will not be garbage-collected but also so that they can be edited in the admin before publishing since there is not yet a post/page editing flow in the Customizer. See #39752.