stream-clone: directly use `_entries_walk` to generate stream-v2 This does not requires that much changes and will give us much more flexibility, like improving revlog handling to gracefully handle race situation.

stream-clone: pre-indent some code This make the next changeset clearer.

local-clone: perform the hardlink/copy based from _entries_walk returns We previously used `_v2_walk`. However it is not bringing us much. So lets use the higher level function instead. This will offer us more flexibility with the `_v2_walk` function… like deleting it eventually.

store: cache the file_size when we get it from disk The point of caching `files` is to ensure consistency and avoiding redoing expensive work. So we cache the file_size once retrieved.