Local autosave: set a temp cookie on submitting the form and change it on redirecting after the post is saved/updated, then use it to determine if saving worked properly. Removes the chance for false positives after saving/updating a post. See #23220

git-svn-id: https://core.svn.wordpress.org/trunk@23693 1a063a9b-81f0-0310-95a4-ce76da25c4cd