Begin a version file upload

Begin uploading a large file in parts. A file may be at most 1 TiB, a provisional limit. The response's file is awaiting_parts, with the upload id, the size of every part but the last, the number of parts, and how many parts the upload accepts at once, starting at the first part not yet hashed. When the request gives a destination, a workspace and a path, the file is staged there once its last part arrives. When the store already has the file, file is already_stored: no parts are needed, and the file is staged at the destination, if one was given. A client whose upload completed without a response, or failed to stage, begins it again.