Expose Async download at Rest API level

A new asynchronous download mechanism has been introduced. It allows getting a status of the asynchronous preparation of the blob to download, before downloading it for real. For instance when calling the operation Blob.BulkDownload answer will first be : {"key":"4db11225407dad8432e75ba2d2778a49","completed":false,"progress":0} Then calling blobstatus enpoint with the provided key {https://nightly.nuxeo.com/nuxeo/nxblobstatus/4db11225407dad8432e75ba2d2778a49} gives an updated result: {"key":"4db11225407dad8432e75ba2d2778a49","completed":true,"progress":100} When completed, one can call the download servlet with the same key: { https://nightly.nuxeo.com/nuxeo/nxbigblob/4db11225407dad8432e75ba2d2778a49 }