scan multiple remotes in one pass

The expensive transfer scan now scans a whole set of remotes in one pass.
So at startup, or when network comes up, it will run only once.

Note that this can result in transfers from/to higher cost remotes being
queued before other transfers of other content from/to lower cost remotes.
Before, low cost remotes were scanned first and all their transfers came
first. When multiple transfers are queued for a key, the lower cost ones
are still queued first. However, this could result in transfers from slow
remotes running for a long time while transfers of other data from faster
remotes waits.

I expect to make the transfer queue smarter about ordering
and/or make it allow multiple transfers at a time, which should eliminate
this annoyance. (Also, it was already possible to get into that situation,
for example if the network was up, lots of transfers from slow remotes
might be queued, and then a disk is mounted and its faster transfers have
to wait.)

Also note that this means I don't need to improve the code in
Assistant.Sync that currently checks if any of the reconnected remotes
have diverged, and if so, queues scans of all of them. That had been very
innefficient, but now doesn't matter.

Assistant/ScanRemotes.hs

@@ -26,21 +26,15 @@ type ScanRemoteMap = TMVar (M.Map Remote ScanInfo)
 newScanRemoteMap :: IO ScanRemoteMap
 newScanRemoteMap = atomically newEmptyTMVar
 
-{- Blocks until there is a remote that needs to be scanned.
- - Processes higher priority remotes first. -}
-getScanRemote :: ScanRemoteMap -> IO (Remote, ScanInfo)
-getScanRemote v = atomically $ do
-	m <- takeTMVar v
-	let l = reverse $ sortBy (compare `on` scanPriority . snd) $ M.toList m
-	case l of
-		[] -> retry -- should never happen
-		(ret@(r, _):_) -> do
-			let m' = M.delete r m
-			unless (M.null m') $
-				putTMVar v m'
-			return ret
+{- Blocks until there is a remote or remotes that need to be scanned.
+ -
+ - The list has higher priority remotes listed first. -}
+getScanRemote :: ScanRemoteMap -> IO [(Remote, ScanInfo)]
+getScanRemote v = atomically $
+	reverse . sortBy (compare `on` scanPriority . snd) . M.toList
+		<$> takeTMVar v
 
-{- Adds new remotes that need scanning to the map. -}
+{- Adds new remotes that need scanning. -}
 addScanRemotes :: ScanRemoteMap -> Bool -> [Remote] -> IO ()
 addScanRemotes _ _ [] = noop
 addScanRemotes v full rs = atomically $ do