Optimized chunked batch upload with S3

Today when we do a chunked batch upload (from Drive typically) on a Nuxeo instance configured with S3 transient stores (which is the common case), we have inefficiencies:

• first each chunk is uploaded form the client to Nuxeo, then saved to S3
• then in BatchFileEntry.getBlob we download all chunks to the filesystem, concatenate them into a blob
• then when this blob is saved (typically in a document) it is uploaded to S3.

This should be optimized to upload avoid doing the concatenation on the Nuxeo side, and instead use S3 Multipart Upload to do the concatenation fully S3-side.

To do that we'd need to add a notion of partial upload to the S3BlobStore (leveraging the BlobWriteContext), then add the same notion to transient stores, and leverage that from the Batch infrastructure.