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/Helper/ChunkedEncryptable.hs

@@ -77,9 +77,11 @@ fileRetriever a k m callback = do
 	a f k m
 	callback (FileContent f)
 
--- A Retriever that generates a L.ByteString containing the Key's content.
-byteRetriever :: (Key -> Annex L.ByteString) -> Retriever
-byteRetriever a k _m callback = callback =<< (ByteContent <$> a k)
+-- A Retriever that generates a lazy ByteString containing the Key's
+-- content, and passes it to a callback action which will fully consume it
+-- before returning.
+byteRetriever :: (Key -> (L.ByteString -> Annex Bool) -> Annex Bool) -> Retriever
+byteRetriever a k _m callback = a k (callback . ByteContent)
 
 {- The base Remote that is provided to chunkedEncryptableRemote
  - needs to have storeKey and retreiveKeyFile methods, but they are