Push uploader control flow into objectstore package

This is follow-up to !602 (merged), and it wraps up my mini-project to simplify our object storage uploader code.

In this MR we:

• replace calls to Write, Close and CloseWithError with a single call to Consume
• remove ETag from the uploader interface
• get rid of the io.Pipe and io goroutine

Closes #294 (closed)