storage: Allow extid reading with filter on extid version

It's used solely with the mercurial loader (for now). Which is currently in charge of filtering out data itself (only version 1 is currently kept). It's planned to be used in other loaders though (git comes to mind). So might as well make it be done directly server side. That will account for less data to pass through wires (serial/deserialization). So all in all making it faster.