Fix saving of postmeta in scheduled changesets

[westonruter]

When a scheduled changeset publishes via WP Cron, the Customizer will bootstrap after the init action. In the course of normal Customizer bootstrapping, it happens at the plugins_loaded action. Because postmeta currently is registered at the init action, this means that the postmeta will not get registered during WP Cron because the Customizer will bootstrap after init.

This is also a problem for changesets that get published via WP-CLI, since init also is done before wp post update $changeset_post_id --post_status=publish.

In the Customizer the way to reproduce the problem is:

1. Open Customizer
2. Open a post to edit.
3. Change the post title
4. Change the featured image
5. Schedule the changes to go live.
6. At the scheduled time, the post title change is published whereas the featured image change is not.