Deprecate then's third argument

Using `then`'s third argument for progress has always been weird.  It's also not in either Promises/A+ or ES6 (it was only optional in Promises/A).  Now that we have `promise.progress`,  I think we should deprecate `then`'s third argument, and steer people to use `promise.progress` instead.

Note that this is not "deprecate the whole of progress functionality as we know it today" (though it may be a first step in that direction).  Progress will continue to function as it does now. I'm just proposing that it's better to have only one API for it, and that API should be separate from `then`.