extend transferrer protocol to send progress bar total size updates

New protocol is not back-compat with old one, but it's never been
released so that's ok.

doc/todo/stalldetection_progress_bar_lacks_ETA.mdwn

@@ -3,3 +3,5 @@ communicates results in a progress display w/o ETA sometimes.
 In particular, it seems to happen downloading from ssh, when the key does
 not have a size. Normally, the size is learned during download and used in
 the progress bar, but somehow this does not happen. --[[Joey]]
+
+> [[fixed|done]] --[[Joey]]