When modifying terms assigned to an object, replace full term recounts with incrementing/decrementing the count as appropriate. This provides a significant performance boost on sites with a high number of term/object relationships and/or posts.
Introduces the functions `wp_increment_term_count()`, `wp_decrement_term_count()`, `wp_modify_term_count_by()` and `wp_modify_term_count_by_now()` for updating the term count.
Introduces the function `_wp_prevent_term_counting()` for preventing double counting on posts that are about to transition.
Adds the parameter `update_count_by_callback` to `register_taxonomy()` to allow developers to use a custom callback for incrementing or decrementing a term count.
Props boonebgorges, davidbaumwald, hellofromTonya, johnbillion, lcyh78, mattoperry, peterwilsoncc, rebasaurus, whyisjake.
Fixes#40351.
git-svn-id: https://develop.svn.wordpress.org/trunk@49141 602fd350-edb4-49c9-b593-d223f7449a82
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