update_rss_feed: use DiskCache to store feed data

classes/cache/adapter.php

@@ -9,6 +9,11 @@ interface Cache_Adapter {
 	 * @return int|false -1 if the file doesn't exist, false if an error occurred, size in bytes otherwise
 	 */
 	public function get_size(string $filename);
+
+	/**
+	 * @return int|false -1 if the file doesn't exist, false if an error occurred, timestamp otherwise
+	 */
+	public function get_mtime(string $filename);
 	/**
 	 * @param mixed $data
 	 *

classes/cache/local.php

@@ -2,6 +2,10 @@
 class Cache_Local implements Cache_Adapter {
 	private string $dir;
 
+	public function get_mtime(string $filename) {
+		return filemtime($this->get_full_path($filename));
+	}
+
 	public function set_dir(string $dir) : void {
 		$this->dir = Config::get(Config::CACHE_DIR) . "/" . basename(clean($dir));
 	}