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.


Built from https://develop.svn.wordpress.org/trunk@47938


git-svn-id: http://core.svn.wordpress.org/trunk@47711 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-object-cache.php

@@ -302,6 +302,27 @@ class WP_Object_Cache {
 		return false;
 	}
 
+	/**
+	 * Retrieves multiple values from the cache.
+	 *
+	 * @since  5.5.0
+	 *
+	 * @param array $keys        Array of keys to fetch.
+	 * @param bool  $force       Optional. Unused. Whether to force a refetch rather than relying on the local
+	 *                           cache. Default false.
+	 *
+	 * @return array Array of values organized into groups.
+	 */
+	public function get_multiple( $keys, $group = 'default', $force = false ) {
+		$values = array();
+
+		foreach ( $keys as $key ) {
+			$values[ $key ] = $this->get( $key, $group, $force );
+		}
+
+		return $values;
+	}
+
 	/**
 	 * Increments numeric cache item's value.
 	 *