Review JSON API module for core-readiness

Per #2757967: API-first initiative, we'd like to move the JSON API module into Drupal 8.3/8.4. For that to happen, the module needs to be in a excellent shape. (It looks like it already is though!)

We need:

1. very solid test coverage
2. since this is following a spec (http://jsonapi.org/format/): compliance with the spec, test coverage verifying that compliance, and code comments linking to the spec where relevant
3. compliance with core best practices wrt naming, structure

Why do this now, while there is not even a Drupal core issue to add this to Drupal core? Because if we do this now, before the JSON API contrib module enters the beta phase, we minimize (and hopefully eradicate) the need for BC breaks, which will make it easier for those sites who are using the contrib module to upgrade to a Drupal core version that includes this module.

Wim Leers has taken on this review.

After the initial review above, and having finished most of it (by January 9, 2017), it's time to look at the exact requirements that must be met. See https://www.drupal.org/core/experimental#requirements.

• There are known architectural issues, for which there are follow-ups — see #2842148: The path for JSON API to "super stability"
• All coding standards are met.
• There are no known security or data integrity issues.
• There's no disruption of stable functionality elsewhere in Drupal.
• There is functional test coverage for the primary use case, with no test failures.
• There is no basic API documentation — because the JSON API module offers no PHP API. It offers an API via routes+controllers+responses, i.e. it offers a HTTP API, not a PHP API. (This is similar to BigPipe, see #2835604: BigPipe provides functionality, not an API: mark all classes & interfaces @internal.) Regarding how to use this HTTP API, there is http://jsonapi.org/format/ + http://jsonapi.org/examples/ + https://www.youtube.com/playlist?list=PLZOQ_ZMpYrZsyO-3IstImK1okrpfAjuMZ
• There are no user interfaces, so there's no need for planning/validating them.
• All critical issues have been resolved, most notably #2829398: Clean up JsonApiResource: the annotation, plugin type, plugin manager, plugin implementations, the dynamic routes generator and the request handler.
• Follow up issues documented and filed for outstanding work: see #2842148: The path for JSON API to "super stability"
• Timeframe for completing the remaining work: the goal is to complete all remaining work by Drupal 8.4, so that it can be marked beta (or even stable) in 8.4, and to flush out remaining problems by 8.5, at which point it should be rock-solid.