handle lastUpdateTime for Wopi implementation

* get the last updated time from the stored object's storage
* improve perf on loading content from stored object's storage: keep the
response in cache

src/Bundle/ChillDocStoreBundle/Service/StoredObjectManagerInterface.php

@@ -12,9 +12,12 @@ declare(strict_types=1);
 namespace Chill\DocStoreBundle\Service;
 
 use Chill\DocStoreBundle\Entity\StoredObject;
+use DateTimeInterface;
 
 interface StoredObjectManagerInterface
 {
+    public function getLastModified(StoredObject $document): DateTimeInterface;
+
     /**
      * Get the content of a StoredObject.
      *