Use wp_cache_add() instead of wp_cache_set() when priming the object term...

Use wp_cache_add() instead of wp_cache_set() when priming the object term cache in update_object_term_cache(). Pass the real post_type to clean_object_term_cache() instead of hard-coding post. Call clean_object_term_cache() from clean_bookmark_cache(). Props leewillis77. fixes #19690

git-svn-id: https://core.svn.wordpress.org/trunk@19729 1a063a9b-81f0-0310-95a4-ce76da25c4cd