implement pausing of transfers

A paused transfer's thread keeps running, keeping the slot in use.
This is intentional; pausing a transfer should not let other
queued transfers to run in its place.

Assistant/TransferQueue.hs

@@ -56,6 +56,7 @@ stubInfo f r = TransferInfo
 	, transferRemote = Just r
 	, bytesComplete = Nothing
 	, associatedFile = f
+	, transferPaused = False
 	}
 
 {- Adds transfers to queue for some of the known remotes. -}