add reservoir sampling algorithm

Summary: Implemented reservoir sampling. The sampling algorithm works on a variety of backends. We probably won't need more than what I've implemented so far, namely a slice-based storage and a persistent version using a storage.Engine.

Test Plan: make test

Reviewers: andybons, spencerkimball

Reviewed By: spencerkimball

Subscribers: team

Differential Revision: http://phabricator.cockroachdb.org/D101