Multi-part upload against the filesystem provider should return ETag similar to S3

When performing a multi-part upload in S3, the resulting ETag is not a content hash, but rather a hash of the content hashes of the parts, followed by "-<number of parts>". It would be great for the jclouds filesystem provider to have the same behavior if it's trying to be an emulation for S3 (or Swift, which has a similar behavior for its Static Large Objects).