Allow WP_Query to preload post data, and meta in wc_get_products()

[leewillis77]

wc_get_products() runs a WP_Query to fetch the relevant post IDs only, and then (as long as the user hasn't specifically requested just the IDs) loads each post / product individually.

This results in a significant increase in the number of database queries executed. This change allows WP_Query to fetch the post & meta information as well, dramatically reducing the number of queries executed.

The 'fetch-IDs-only' optimisation to WP_Query is retained as long as the user has asked for only IDs to be returned from wc_get_products().

With the following test script on my dev box:

<?php

$args = array(
	'status'      => array( 'publish' ),
	'limit'       => 20,
	'offset'      => 0,
	'return'      => 'objects',
);
wc_get_products($args);

Before this change, the code above executes 158 database queries. After this PR that is reduced to 100 queries.

If you combine it with the changes in #12288 then that number comes down to 42 queries.

[leewillis77]

Some more context. I'm porting the Google Product Feed extensions to use wc_get_products() and these two PRs reduce the queries on my test setup from 1,478 down to 457.

There's still an issue somewhere though as the current version only runs 274 queries. I'll keep investigating.