Make sure all field returns use filters in WP Query

[spacedmonkey]

Currently if fields is set to either id or id=>parent, it doesn't run through all the filters as these return early.

[spacedmonkey]

In 41678.diff code has be refactored to run through more filters.

Key features

• There are now 2 only types of query, id and all fields. id=>parents are now treated the same as all fields and the return is changed at the latest possible moment.
• New filter added named posts_results_ids for query of 'ids', to get a filter of result of query. Useful for caching plugins. This new filter is added, as posts_results expects an array of WP_Post objects.
• 'id=>parents' now runs through the following filters the_posts, posts_results, posts_request_ids and split_the_query.

This patch will be extremely useful for a plugin like elasticpress.

[spacedmonkey]

Related: #22176