Split Buffer into WriteBuffer/ReadBuffer.

service/datastore/serialize/serialize_test.go

Index: service/datastore/serialize/serialize_test.go
diff --git a/service/datastore/serialize/serialize_test.go b/service/datastore/serialize/serialize_test.go
index 55e9caafa83520a5a6f07ff23ed8e05c8695ef62..24607ed1c8f1ea2eb7479edb074b41f82e1f4ab4 100644
--- a/service/datastore/serialize/serialize_test.go
+++ b/service/datastore/serialize/serialize_test.go
@@ -37,7 +37,7 @@ func mkKey(appID, namespace string, elems ...interface{}) *ds.Key {
 	return ds.MkKeyContext(appID, namespace).MakeKey(elems...)
 }
 
-func mkBuf(data []byte) Buffer {
+func mkBuf(data []byte) WriteBuffer {
 	return Invertible(bytes.NewBuffer(data))
 }