Add the Blob.batchId attribute

Currently, the way operations are used to attach a blob to a document (using Blob.AttachOnDocument or FileManager.IMport operations for example) is not really clean. It relies on an inexistent attribute Blob.batch_id set either from the uploader either from Nuxeo Drive; after the Blob creation.

Blob should have a default batchId attribute and relying only on it.

Backward compatibility will be kept until version 4.0.