Remove some term cache misses

[johnbillion]

On post listing screens a database query is performed for each taxonomy on each post. We can hit the term cache instead by using get_the_terms() in place of wp_get_object_terms() and wp_get_post_terms().

By default, this change won't impact performance because the term cache isn't primed at this point, but on a site using a persistent object cache it'll hit the term cache each time instead of performing a database query. This saves {numberofposts} × {numberoftaxonomies} queries which can be quite a few on custom post type screens which display several taxonomy columns.

Patch.

I don't think using get_the_terms() in get_terms_to_edit() is a good idea.

Checking the cache directly in get_terms_to_edit() maybe, although it probably won't help much.

I don't know if using get_the_terms() here is a bad idea. While it is a front-end template tag, we already use get_the_tags() and get_the_category() in the list tables.

That said, if it's decided we don't do this, then we should be checking get_object_term_cache() before hitting wp_get_post_terms().