Fix panic on network timeout during push

`Upload` already closes the reader returned by `compress` and the
progressreader passed into it, before returning. But even so, the
io.Copy inside compress' goroutine needs to attempt a read from the
progressreader to notice that it's closed, and this read has a side
effect of outputting a progress message. If this happens after `Upload`
returns, it can result in a write to a closed channel. Change `compress`
to return a channel that allows the caller to wait for its goroutine to
finish before freeing any resources connected to the reader that was
passed to it.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>