resumable uploads

Hi,

I think I've left this feedback before, but I'll reiterate. I think this
approach is correct. This is what you need to do to get a big photo or
video off of a cell phone. The name of the draft suggests that you upload
the second half later, like maybe after a few hours. No. This is what you
must do to upload anything, in the sense that if you don't do something
like this, you will have a high failure rate.

The authors have URLSession, which should make everyone fall in line. Since
everything is TLS now, we can have nice things, and use somewhat clever
HTTP codes.

thanks,
Rob