Add support for flushing term cache when objects terms are modified

[MosheGross]

As of now, lets say i query
/wp-json/wp/v2/categories
I get back the count for each category (and by default it also hides empty categories).

If I add a post or edit a posts category, the counts should change, but they don't.

So I added function to hook into set_object_terms.

I added the filter wp_rest_cache/flush_on_set_terms and defaulted it to false for performance, as the function is quite heavy, it has to do the following

• Get term ids from taxonomy ids
• Get all term ids for object
• Get all parent term ids if the taxonomy is hierarchical
• Loop over each affected term (including ancestors) and clear related caches

[MosheGross]

@richardkorthuis can you have a look pls

[richardkorthuis]

Hi @MosheGross

Sorry for the late reply, but I finally had time to check your PR.

I am going to close your PR, not because I don't like it, but because I have implemented your changes with some small improvements, for instance some code that didn't completely adhere to the WP Coding Standards and also I made the wp_rest_cache/flush_on_set_terms to default to true. We just released a new version of our plugin with this improvement in it.

Thank you for your contribution!

[MosheGross]

@richardkorthuis amazing, thanks