Problems deleting image files from the images backend

The merging and deleting tools use the ``delete_all_files_for_image`` function
https://github.com/c2corg/v6_api/blob/81acc5a87544da06ec5c6a086aa2297d5448f868/c2corg_api/views/image.py#L116-L142
to remove image files from the images backend.

Testing in my instance, I have realized that the documents were actually merged/dropped but that the image files were still available after proceeding.

Steps to reproduce:
* open an image document
* middle-click on the image to open the file in a new tab
* delete or merge the image document => success
* try to reload the image file in the second tab => still accessible

For instance http://sos.exo.io/c2corg_demov6_active/1484838792_63859547.jpg should have been deleted.

Is it an API or an images backend issue?

Start in c2corg_api/views/image.py, especially delete_all_files_for_image at lines 116-142, and reproduce the deletion or merge flow described in the issue. Trace whether the API call reaches the images backend; done means the document is removed and its previously opened image URL is no longer accessible.

Written by the indexing model from the issue text.