delta-find: introduce and use specialized _DeltaSearch class For now, we introduce some very simple variant, but they are still useful to display how having the class can helps keeping the simple case simple and their special case out of more advanced logic.

delta-find: introduce a base class for _DeltaSearch This prepare the introduction of specialized the class in the next changesets.

delta-find: simplify the delta checking function for snapshot Since the function is all about snapshot, we can safely use an early return and make the result simpler.