statsd: restore support for {influx,librato,stackdriver} backends

These packages will be placed into an environment using
`backendsToPackages`. This function explicitly maps backends to
`pkgs.nodePackages.${type}` unless it's a builtin. This ensures that only
valid backends that work on NixOS are used (if not, the build already
breaks at evaluation time).

The log will be redirected to `stdout` to be able to watch the entire
output using `journalctl`.

Configuration parameters for the backends need to be set using
`services.statsd.extraConfig` as each backend has its own options and
all of them shouldn't be validated and checked explicitly and manually.