Split the main WP_Query posts query into two queries to avoid temp tables....

Split the main WP_Query posts query into two queries to avoid temp tables. Leverage cache to avoid second query in persistent cache environments. Props scribu, cheald, prettyboymp. see #18536

git-svn-id: https://core.svn.wordpress.org/trunk@19918 1a063a9b-81f0-0310-95a4-ce76da25c4cd