stream-clone: introduce a _entries_walk...
stream-clone: introduce a _entries_walk
That function insert itself between the store and `_v2_walk`. It only deals with
StoreEntry unlike `_v2_walk` that deal with actual file.
A share of the `_v2_walk` logic will be moved in this new `_entry_walk`
function. Having this function will help us to implement a new "v3" version of
the protocol that will be more entry centric.