Use microtime() instead of incrementors for last_changed to to avoid race conditions with cache evictions.

Props westi
fixes #23448


git-svn-id: http://core.svn.wordpress.org/trunk@23401 1a063a9b-81f0-0310-95a4-ce76da25c4cd
TinyMCE: include https://github.com/tinymce/tinymce/commit/cd84a63d4addf27c60ef32dcbe1a49bbee30150a to fix backspace and line breaks issues in WebKit, see #23337

git-svn-id: http://core.svn.wordpress.org/trunk@23402 1a063a9b-81f0-0310-95a4-ce76da25c4cd