S3: support chunking

The assistant defaults to 1MiB chunk size for new S3 special remotes.
Which will work around a couple of bugs:
  http://git-annex.branchable.com/bugs/S3_memory_leaks/
  http://git-annex.branchable.com/bugs/S3_upload_not_using_multipart/

Types/Key.hs

@@ -15,6 +15,7 @@ module Types.Key (
 	file2key,
 	nonChunkKey,
 	chunkKeyOffset,
+	isChunkKey,
 
 	prop_idempotent_key_encode,
 	prop_idempotent_key_decode
@@ -62,6 +63,9 @@ chunkKeyOffset k = (*)
 	<$> keyChunkSize k
 	<*> (pred <$> keyChunkNum k)
 
+isChunkKey :: Key -> Bool
+isChunkKey k = isJust (keyChunkSize k) && isJust (keyChunkNum k)
+
 fieldSep :: Char
 fieldSep = '-'