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.

Types/StoreRetrieve.hs

@@ -30,6 +30,6 @@ isByteContent (FileContent _) = False
 type Storer = Key -> ContentSource -> MeterUpdate -> Annex Bool
 
 -- Action that retrieves a Key's content from a remote, passing it to a
--- callback.
+-- callback, which will fully consume the content before returning.
 -- Throws exception if key is not present, or remote is not accessible.
 type Retriever = Key -> MeterUpdate -> (ContentSource -> Annex Bool) -> Annex Bool