better byteRetriever

Make the byteRetriever be passed the callback that consumes the bytestring.

This way, there's no worries about the lazy bytestring not all being read
when the resource that's creating it is closed.

Which in turn lets bup, ddar, and S3 each switch from using an unncessary
fileRetriver to a byteRetriever. So, more efficient on chunks and encrypted
files.

The only remaining fileRetrievers are hook and external, which really do
retrieve to files.

Remote/Directory/LegacyChunked.hs

@@ -94,14 +94,14 @@ retrieve locations d basek a = do
 	tmpdir <- fromRepo $ gitAnnexTmpMiscDir
 	createAnnexDirectory tmpdir
 	let tmp = tmpdir </> keyFile basek ++ ".directorylegacy.tmp"
-	a $ Just $ byteRetriever $ \k -> liftIO $ do
-		void $ withStoredFiles d locations k $ \fs -> do
+	a $ Just $ byteRetriever $ \k sink -> do
+		liftIO $ void $ withStoredFiles d locations k $ \fs -> do
 			forM_ fs $
 				S.appendFile tmp <=< S.readFile
 			return True
-		b <- L.readFile tmp
-		nukeFile tmp
-		return b
+		b <- liftIO $ L.readFile tmp
+		liftIO $ nukeFile tmp
+		sink b
 
 checkPresent :: FilePath -> (FilePath -> Key -> [FilePath]) -> Key -> Annex (Either String Bool)
 checkPresent d locations k = liftIO $ catchMsgIO $