[HttpKernel] Changed the directory name in which Store stores data.

Previously, it used this pattern:

md/abcd/123456789

now it uses this one :

md/ab/cd/123456789

src/Symfony/Component/HttpKernel/Cache/Store.php

@@ -339,7 +339,7 @@ class Store
 
     public function getPath($key)
     {
-        return $this->root.DIRECTORY_SEPARATOR.substr($key, 0, 2).DIRECTORY_SEPARATOR.substr($key, 2, 4).DIRECTORY_SEPARATOR.substr($key, 4);
+        return $this->root.DIRECTORY_SEPARATOR.substr($key, 0, 2).DIRECTORY_SEPARATOR.substr($key, 2, 2).DIRECTORY_SEPARATOR.substr($key, 4, 2).DIRECTORY_SEPARATOR.substr($key, 6);
     }
 
     /**