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.

Built from https://develop.svn.wordpress.org/trunk@27635


git-svn-id: http://core.svn.wordpress.org/trunk@27478 1a063a9b-81f0-0310-95a4-ce76da25c4cd