Make view use memory persistence, remove obsolete code.

This makes the memory persistence the backing store for views and
adjusts the MetricPersistence interface accordingly. It also removes
unused Get* method implementations from the LevelDB persistence so they
don't need to be adapted to the new interface. In the future, we should
rethink these interfaces.

All staleness and interpolation handling is now removed from the storage
layer and will be handled only by the query layer in the future.
Merge pull request #160 from prometheus/julius-refactor-persistence

Make view use memory persistence, remove obsolete code.