assistant: Add 1/200th second delay between checking each file in the full transfer scan, to avoid using too much CPU.

The slowdown is not going to be large in typical small-ish repos.
And it does not seem to matter if the assistant reacts a little bit slower
in situations involving the expensive scan, since:

a) Those situations typically involve getting back in sync after something
   has changed on a remote, often after a disconnect of some duration.
   So taking a few seconds more is not noticable.
b) If the scan finds things that it needs to do, it will start
   blocking anyway after 10 transfers are queued (due to use of
   queueTransferWhenSmall). So, only the speed of finding the first 10
   transfers will be impacted by this change.

This commit was sponsored by Jochen Bartl on Patreon.

Assistant/Threads/TransferScanner.hs

@@ -25,6 +25,7 @@ import qualified Types.Remote as Remote
 import Utility.ThreadScheduler
 import Utility.NotificationBroadcaster
 import Utility.Batch
+import Utility.ThreadScheduler
 import qualified Git.LsFiles as LsFiles
 import Annex.WorkTree
 import Annex.Content
@@ -32,6 +33,7 @@ import Annex.Wanted
 import CmdLine.Action
 
 import qualified Data.Set as S
+import Control.Concurrent
 
 {- This thread waits until a remote needs to be scanned, to find transfers
  - that need to be made, to keep data in sync.
@@ -145,6 +147,10 @@ expensiveScan urlrenderer rs = batch <~> do
 			(findtransfers f unwanted)
 				=<< liftAnnex (lookupFile f)
 		mapM_ (enqueue f) ts
+
+		{- Delay for a short time to avoid using too much CPU. -}
+		liftIO $ threadDelay $ fromIntegral $ oneSecond `div` 200
+
 		scan unwanted' fs
 
 	enqueue f (r, t) =