Customize: Selectively merge settings from autosave revisions

[westonruter]

In follow-up on #42024, an autosave revision is created as part of changeset locking. When a user returns to the customizer after a lock has been lifted, they will be prompted to restore their revision. The restoration logic should only load settings from the changeset which are older than the settings in the customize_changeset post (see new setting prop date_modified_gmt), or the settings for which there is no existing setting in the changeset. This merge will more intelligently ensure that a user's restored autosave revision won't override the changes another user saved. There may be some cases, however, where selectively restoring parts of an autosave will have unexpected results (e.g. opting to include a newly-created widget, but not accepting the change to the sidebar), so more investigation will be needed.

The problem of conflict resolution in the Customizer is a large problem which is also being worked on in the ​Customize Snapshots plugin.

See also #31436 (Handle conflicts in concurrent Customizer sessions)