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/DaemonStatus.hs

@@ -183,17 +183,19 @@ adjustTransfersSTM dstatus a = do
 	s <- takeTMVar dstatus
 	putTMVar dstatus $ s { currentTransfers = a (currentTransfers s) }
 
-{- Updates a transfer's info. Preserves any transferTid value, which is not
- - written to disk. -}
+{- Updates a transfer's info.
+ - Preserves the transferTid and transferPaused values,
+ - which are not written to disk. -}
 updateTransferInfo :: DaemonStatusHandle -> Transfer -> TransferInfo -> IO ()
 updateTransferInfo dstatus t info =
 	notifyTransfer dstatus `after` modifyDaemonStatus_ dstatus go
 	where
 		go s = s { currentTransfers = update (currentTransfers s) }
 		update m = M.insertWith' merge t info m
-		merge new old = case transferTid old of
-			Nothing -> new
-			Just _ -> new { transferTid = transferTid old }
+		merge new old = new
+			{ transferTid = maybe (transferTid new) Just (transferTid old)
+			, transferPaused = transferPaused new || transferPaused old
+			}
 
 {- Removes a transfer from the map, and returns its info. -}
 removeTransfer :: DaemonStatusHandle -> Transfer -> IO (Maybe TransferInfo)