Term query endpoints should use `WP_Term_Query`

[boonebgorges]

Original ticket: ​https://github.com/WP-API/WP-API/issues/2740.

A PR was approved on WP-API for this ticket, but was blocked by the core merge. Now that the merge is done, work can continue here. ​https://github.com/WP-API/WP-API/pull/2780

[boonebgorges]

38504.diff​ ports the PR to trunk. A few variable names and other miscellany changed in the interim, so I'd like a review. Ping @rachelbaker or @joehoyle

[boonebgorges]

In the interest of making beta, I'm going to rereview and commit this. It is only a minor modification to a previously approved PR. If there are problems, we'll fix them as bugs.

[boonebgorges]

In 38974:

REST API: Use wp_get_object_terms() when fetching terms for a post object.

The WP-API plugin originally used a custom method for fetching object
terms in a way that supported the object cache and also accepted all
parameters for get_terms(). In [38667], the internals of
wp_get_object_terms() were modified to use WP_Term_Query, thus
delivering in a native fashion the features that the API had
previously achieved bespokely.

Fixes #38504.