Several important cleanups and deprecations.

EachFunc is deprecated.

Remove deprecated ``Pair`` and ``GetAll``.

These were originally used for forensic and the old gorest impl.
Nothing today in the user-facing path nor the tests uses them,
especially since the advent of the ForEach protocol in the
interface.
Merge pull request #94 from prometheus/refactor/persistence/interface-cleanups

Remove Cruft from Storage Interfaces