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/class-wp-object-cache.php

@@ -290,6 +290,20 @@ class WP_Object_Cache {
 		return $values;
 	}
 
+	/**
+	 * Removes all cache items in a group.
+	 *
+	 * @since 6.1.0
+	 *
+	 * @param string $group Name of group to remove from cache.
+	 * @return true Always returns true.
+	 */
+	public function flush_group( $group ) {
+		unset( $this->cache[ $group ] );
+
+		return true;
+	}
+
 	/**
 	 * Retrieves the cache contents, if it exists.
 	 *