add a progress callback to storeKey, and threaded it all the way through

Transfer info files are updated when the callback is called, updating
the number of bytes transferred.

Left unused p variables at every place the callback should be used.
Which is rather a lot..

Command/RecvKey.hs

@@ -25,7 +25,7 @@ seek = [withKeys start]
 start :: Key -> CommandStart
 start key = ifM (inAnnex key)
 	( error "key is already present in annex"
-	, fieldTransfer Download key $ do
+	, fieldTransfer Download key $ \p -> do
 		ifM (getViaTmp key $ liftIO . rsyncServerReceive)
 			( do
 				-- forcibly quit after receiving one key,