Wrap dto.SampleKey with business logic type.

The curator work can be done easier if dto.SampleKey is no longer
directly accessed but rather has a higher level type around it that
captures a certain modicum of business logic.  This doesn't look
terribly interesting today, but it will get more so.
Merge pull request #166 from prometheus/refactor/readability/business-logic-types

Wrap dto.SampleKey with business logic type.