Simplifications of GetFingerprintsForLabelSet.

``MetricPersistence.GetFingerprintsForLabelSet(s *model.LabelSet)`` ->
``MetricPersistence.GetFingerprintsForLabelSet(s model.LabelSet)``.
Merge pull request #73 from prometheus/refactor/storage/simplifications

Simplifications of GetFingerprintsForLabelSet.