statsd: updated package and nixos service

* package statsd node packages separatly since they actually require
  nodejs-0.10 or nodejs-0.12 to work (which is ... well old)

* remove statsd packages and its backends from "global" node-packages.json.
  i did not rebuild it since for some reason npm2nix command fails. next time
  somebody will rerun npm2nix statsd packages are going to be removed.

* statsd service: backends are now provided as strings and not anymore as
  packages.