User:Bryan/UploadFromUrlJob review

Putting my notes here, but they refer to upload-by-url as of r65463. I reviewed the entire file, not the diffs, so the fixmes may very well have been introduced in other revisions by me or michael.
 * upload-by-url via the API is only possible async. I think that be default the upload must be synchronous (easier for clients and no b/w compat breaking changes) and have a parameter to make it async.