GetFingerprintsForLabelName simplifications.

``MetricPersistence.GetFingerprintsForLabelName(l *model.LabelName)``
->
``MetricPersistence.GetFingerprintsForLabelName(l model.LabelName)``
Interface simplifications: GetValueAtTime().

Pointer arguments to copies.
Interface simplifications.

GetBoundaryValues() from pointers to values.
Interface simplifications: GetRangeValues

From pointers to copies.
Merge pull request #74 from prometheus/refactor/storage/simplifications

GetFingerprintsForLabelName simplifications.