[rfc] transports to pass progress messages

[John Arbash Meinel]

Martin Pool wrote:
> Summary: Transports should have a connection to the ui progress
> indicator, and tell it when they read/write data, so that we get a
> bottom-up progress indicator, as well as the current top-down one.
> 

Seems reasonable. I would be a little concerned about implementation, 
since you are likely to have at least 2 transports operating at any 
given time. Since you are pulling from one, and writing to the other.

Also in some cases they will not be perfectly in sync, read from one, 
think about it a bit, write out something similar but not exactly the same.

I think we *do* want something like this, I just wanted to bring up some 
implementation concerns.

John
=:->