Optimise the capture of existing blobs

When the client requests an upload of objects that exist on the server, casd should handle this efficiently: In the case of a tree where a subset of objects are missing these should be uploaded without requiring upload of the entire tree. It's important that if the client sends a request to upload an existing object it quickly receives confirmation without needing to transfer the blob itself such that multiple requests to upload the same object will be cheap.

relates to BuildStream/buildstream#985 (closed) (see BuildStream/buildstream!1563 (comment 218641761))