Fix thumbnail and renditions for assets already moved to Cold Storage

Pre-requisite: install the nuxeo-coldstorage and configure the instance to use AWS S3 storage

Steps to reproduce:

1. upload the file FILE1 with the batch upload
2. create a new Picture document and use the previous batch upload to attach the file FILE1
3. observe that the document is created and after a while a thumbnail and renditions are computed, and a preview is visible
4. send the document to Cold Storage
5. observe that a message is displayed to confirm and a rendition is now used as the main content
6. upload the same file FILE1 with a new batch upload
7. create a new Picture document and use this batch upload to attach the file FILE1
8. observe that the new document is created and you see immediately that the document is in Cold Storage (because using a file already sent to Cold Storage)
9. observe also that the preview display an empty image and no rendition is attached to the document ===> this is the bug

Expected behavior: after creating a new document whose main blob is already in Cold Storage

• Create the new document and put the blob in regular S3 storage
• Delete existing blob that has the cold storage storage class (avoids having to do a retrieve which would be costly)

Known limitation:
Existing document(s) will still display as being in cold storage, but using the retrieval / restore option on them does not actually involve any cost in this scenario.
This tradeoff was chosen because keeping all documents up to date would have a big impact on performance for every document created or updated, as it would require to query all documents in the repository.