Defer term counts during multiple inserts

[tellyworth]

wp_update_term_count() gets called any time a post is inserted with tags or categories. It can cause several queries and cache invalidations per post insert, which adds up quickly during an import.

The enclosed patch provides a wp_defer_term_counting() function that can be used to temporarily disable term counts when inserting multiple posts:

wp_defer_term_counting(true);
do_many_inserts();
wp_defer_term_counting(false);

Any deferred counts will be automatically updated when wp_defer_term_counting(false) is called. My testing shows this saves about 6500 queries on a 500 post import.

The patch doesn't touch the import code yet, I wanted to post it for separate review first because it's probably useful elsewhere.

[tellyworth]

The second patch defers counting when importing a wxr file.

[ryan]

(In [6376]) Defer term counting during import. Props tellyworth. fixes #5377