Migrate cache purge function to a proper class

And update dependencies and tests.

Note that SESSION['tags'] has been removed a log ago

application/feed/Cache.php

@@ -1,38 +0,0 @@
-<?php
-/**
- * Cache utilities
- */
-
-/**
- * Purges all cached pages
- *
- * @param string $pageCacheDir page cache directory
- *
- * @return mixed an error string if the directory is missing
- */
-function purgeCachedPages($pageCacheDir)
-{
-    if (! is_dir($pageCacheDir)) {
-        $error = sprintf(t('Cannot purge %s: no directory'), $pageCacheDir);
-        error_log($error);
-        return $error;
-    }
-
-    array_map('unlink', glob($pageCacheDir.'/*.cache'));
-}
-
-/**
- * Invalidates caches when the database is changed or the user logs out.
- *
- * @param string $pageCacheDir page cache directory
- */
-function invalidateCaches($pageCacheDir)
-{
-    // Purge cache attached to session.
-    if (isset($_SESSION['tags'])) {
-        unset($_SESSION['tags']);
-    }
-
-    // Purge page cache shared by sessions.
-    purgeCachedPages($pageCacheDir);
-}