Cache API: Introduce wp_cache_get_multi().

Many caching backend have support for multiple gets in a single request. This brings that support to core, with a compatability fallback that will loop over requests if needed.

Fixes: #20875.
Props: nacin, tollmanz, wonderboymusic, ryan, jeremyfelt, spacedmonkey, boonebgorges, dd32, rmccue, ocean90, jipmoors, johnjamesjacoby, tillkruess, donmhico, davidbaumwald, SergeyBiryukov, whyisjake.



git-svn-id: https://develop.svn.wordpress.org/trunk@47938 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/pluggable.php

@@ -290,6 +290,11 @@ class Tests_Pluggable extends WP_UnitTestCase {
 						'force' => false,
 						'found' => null,
 					),
+					'wp_cache_get_multiple'              => array(
+						'keys',
+						'group' => '',
+						'force' => false,
+					),
 					'wp_cache_incr'                      => array(
 						'key',
 						'offset' => 1,