MDL-58684 files: Ensure stored_file never serializes the file_storage

Magic methods __sleep and __wakeup used to properly store only the
file_record property and to reinit using get_file_storage().

lib/filestorage/stored_file.php

@@ -93,6 +93,24 @@ class stored_file {
         $this->filesystem = $fs->get_file_system();
     }
 
+    /**
+     * Magic method, called during serialization.
+     *
+     * @return array
+     */
+    public function __sleep() {
+        // We only ever want the file_record saved, not the file_storage object.
+        return ['file_record'];
+    }
+
+    /**
+     * Magic method, called during unserialization.
+     */
+    public function __wakeup() {
+        // Recreate our stored_file based on the file_record, and using file storage retrieved the correct way.
+        $this->__construct(get_file_storage(), $this->file_record);
+    }
+
     /**
      * Whether or not this is a external resource
      *