add VerifiedCopy data type

There should be no behavior changes in this commit, it just adds a more
expressive data type and adjusts code that had been passing around a [UUID]
or sometimes a Maybe Remote to instead use [VerifiedCopy].

Although, since some functions were taking two different [UUID] lists,
there's some potential for me to have gotten it horribly wrong.

Assistant/Threads/SanityChecker.hs

@@ -191,7 +191,7 @@ dailyCheck urlrenderer = do
 	void $ liftAnnex $ setUnusedKeys unused
 	forM_ unused $ \k -> do
 		unlessM (queueTransfers "unused" Later k Nothing Upload) $
-			handleDrops "unused" True k Nothing Nothing
+			handleDrops "unused" True k Nothing []
 
 	return True
   where