Make sure any precreated session cookies on the wrong path are deleted (

matomo-org#14026)

* Make sure if session cookie path in INI config is different from path in php.ini that any precreated session cookie on the wrong path is deleted.

* Only do cookie change if not posting.

* Check against final value

* Switch to simpler approach of changed session name, since it needs to be changed anyway.

core/Session.php

@@ -19,7 +19,7 @@
  */
 class Session extends Zend_Session
 {
-    const SESSION_NAME = 'PIWIK_SESSID';
+    const SESSION_NAME = 'MATOMO_SESSID';
 
     public static $sessionName = self::SESSION_NAME;