Create one autosave per user

[azaozz]

Currently when autosaving drafts we overwrite them, when autosaving published posts, we keep one autosaved revision. This works well as long as there is only one post author.

Having per-user autosaves will affect only sites with many authors/editors. It will avoid overwriting when more than one user edits a post, provide better audit trail, and let us auto save data stored in the browser even if another user is editing at that moment.

In 23631:

Use auto-draft rather than post_ID = 0 to detect whether we are adding a new post. see #23665.

In 23665-2.patch​:

• Don't short-circuit wp_get_post_revisions() when WP_POST_REVISIONS is false. It is used to get the autosave(s) too.
• Show the correct preview to each user when several users are editing.

23665-post-name.patch​ short-circuits wp_unique_post_slug() for post revisions. Currently revisions are saved with post_name {$post->post_parent}-revision, and autosaves: {$post->post_parent}-autosave. When there are more than one, wp_unique_post_slug kicks in and adds -2, -3, etc. to the end. This is intended for post slugs and not necessary for revisions. If we remove it, or remove it only for autosaves, we can run a different query to fetch only the autosaves for a post instead of looping through all revisions as in the other patch.

In 23665-3.patch:

• Update wp_ajax_autosave() removing post locks references.
• Update autosave.js removing post locks and logged out warnings.

In 23735:

Create one autosave per user rather than a single autosave for all users. Remove unused code from autosave.js and wp_ajax_autosave(). See #23665.

In 23830:

Fix wp_list_post_revisions() to include all autosaves when listing autosaves only and properly include or exclude the current post, see #23665, see #16215

In 23889:

No need to check for the presence of a current user in a privileged ajax action. see #23665.

In 23890:

Replace $user_id removed in [23889] with a direct function call. see #23665.

Fix logic for generating post preview $url in post_preview()

Introduce wp_get_preview_post_url() to funnel post-preview URL handling through, and replace in all instances. Also patches _set_preview() and moves _show_post_preview() to 'set_current_user' priority 20 instead.