store: add logic to group revlog file together For now each file get its own entry, this will help stopping this, soon™. We use such gathering in the `basicstore` code.

store: change `_walk` return to `(filename, (type, size))` If we are to group file per revlog, having the filename as the "main key" will be useful. This change will make the following changes clearer.

store: lazily get file size on demand for the fncache case We don't have the information in the first place, so we can avoid querying the file system inconditionnaly for use case we don't needs it. This change requires the StoreFile class to be passed a vfs to retrieve the file_size if needed. In the non-fncache case, we already have the information from file system walking, so we keep it and use it.

store: only access is_volatile information through the file object This make sure other code only access this information through the proper API, and it prepare for store entries to be able to agregate multiple files.

store: only access file_size information through the file object This make sure other code only access this information through the proper API, and it prepare for store entries to be able to agregate multiple files.

store: have custom init for entries class This will get useful to add special processing later in this series.