Storage interface to TieredStorage.

This commit drops the Storage interface and just replaces it with a
publicized TieredStorage type.  Storage had been anticipated to be
used as a wrapper for testability but just was not used due to
practicality.  Merely overengineered.  My bad.  Anyway, we will
eventually instantiate the TieredStorage dependencies in main.go and
pass them in for more intelligent lifecycle management.

These changes will pave the way for managing the curators without
Law of Demeter violations.
Merge pull request #215 from prometheus/refactor/storage/publicization

Storage interface to TieredStorage.