WP_Query: allow split_the_query = false to avoid a split.

see #26937.


git-svn-id: https://develop.svn.wordpress.org/trunk@27633 602fd350-edb4-49c9-b593-d223f7449a82
Return a bool from wp_using_ext_object_cache(), never a null.

see #26937.


git-svn-id: https://develop.svn.wordpress.org/trunk@27634 602fd350-edb4-49c9-b593-d223f7449a82
Fix various issues with WP_Adjacent_Post:
 * Performance / number of queries.
 * Incorrect results caused by sticky posts.
 * Back compat for filters, which had used "WHERE" while WP_Query does not; and fixing table references.

props ethitter.
fixes #26937.


git-svn-id: https://develop.svn.wordpress.org/trunk@27635 602fd350-edb4-49c9-b593-d223f7449a82