Fix missing current principal in DownloadService

Steps to reproduce :

1. contribute a contribution to check the permission during the download

   <extension target="org.nuxeo.ecm.core.io.download.DownloadService" point="permissions">
     <permission name="Read">
       <script language="JavaScript">
         function run() {
           print("----------------- Start check permissions for DownloadService -----------------");
           print("reason " + Reason);
           print("rendition " + Rendition);
           print("doc " + (Document == null ? "null" : Document.getId()));
           print("filename " + (Blob == null ? "null" : Blob.getFilename()));
           print("xpath " + XPath);
           var userName = null;
           if (CurrentUser != null) {
             // CurrentUser can be null during the BulkAction
             userName = CurrentUser.getName();
             print("user " + userName);
           }
           return true;
         }
       </script>
       </permission>
   </extension>
   

1. Navigate to a workspace
2. Select the action "Download All as ZIP"
3. Observe the following traces in the logs

   ----------------- Start check permissions for DownloadService -----------------
   reason download
   rendition null
   doc eb4b4df8-706c-49aa-868d-e4a45bb538a1
   filename CPP.zip
   xpath null
   

   => the current user is not displayed, which means that it's missing to evaluate the permissions when downloading in this case

Expected behavior: it should be possible to check the current principal in any case

The problem is related to the way AsyncOperationAdapter starts a new thread to do the asynchronous (bulk) download.