Use the object service to store artifacts

[djmitche]

This epic involves adding support for a new artifact type that involves using the object service. For the most part, this should refer the caller to the object service, rather than hiding the object service behind the queue API -- that way workers can adopt improvements to the object service without changes to the queue.

That's tricky in two places:

• getArtifact currently assumes a simple download, so let's add new methods to get artifact metadata without downloading it, and a method the caller can use to talk to the object service
• For upload, the queue wants to control the expiration of the artifact. That's not possible with scopes. However, we set up for this already: once startUpload is called, the expiration and uploadId are fixed for that object name. So, queue should call startUpload with its own uploadId and with no available methods, "locking" the uploadId and expiration, and then let the caller call startUpload again, with methods this time.