Cache API: Add wp_cache_flush_group function.

Add a new plugable function called `wp_cache_flush_group`, that will allow developers to clear whole cache groups with a single call. Developers can detect if their current implementation of an object cache supports flushing by group, by calling `wp_cache_supports_group_flush` which returns true if it is supported. If the developers of the object cache drop-in has not implemented `wp_cache_flush_group` and `wp_cache_supports_group_flush`, these functions are polyfilled and `wp_cache_supports_group_flush` defaults to false.

Props Spacedmonkey, filosofo, ryan, sc0ttkclark, SergeyBiryukov, scribu, Ste_95, dd32, dhilditch, dougal, lucasbustamante, dg12345, tillkruess, peterwilsoncc, flixos90, pbearne.
Fixes #4476.

git-svn-id: https://develop.svn.wordpress.org/trunk@53763 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/cache.php

@@ -22,6 +22,19 @@ function wp_cache_init() {
 	$GLOBALS['wp_object_cache'] = new WP_Object_Cache();
 }
 
+/**
+ * Whether the object cache implementation supports flushing individual cache groups.
+ *
+ * @since 6.1.0
+ *
+ * @see WP_Object_Cache::flush_group()
+ *
+ * @return bool True if group flushing is supported, false otherwise.
+ */
+function wp_cache_supports_group_flush() {
+	return true;
+}
+
 /**
  * Adds data to the cache, if the cache key doesn't already exist.
  *
@@ -281,6 +294,25 @@ function wp_cache_flush_runtime() {
 	return wp_cache_flush();
 }
 
+/**
+ * Removes all cache items in a group, if the object cache implementation supports it.
+ * Before calling this method, always check for group flushing support using the
+ * `wp_cache_supports_group_flush()` method.
+ *
+ * @since 6.1.0
+ *
+ * @see WP_Object_Cache::flush_group()
+ * @global WP_Object_Cache $wp_object_cache Object cache global instance.
+ *
+ * @param string $group Name of group to remove from cache.
+ * @return bool True if group was flushed, false otherwise.
+ */
+function wp_cache_flush_group( $group ) {
+	global $wp_object_cache;
+
+	return $wp_object_cache->flush_group( $group );
+}
+
 /**
  * Closes the cache.
  *