Expand LRU cache to support FIFO and arbitrary eviction decision making. Also added an additional llrb backing store to support binary searches in the cache.

Test Plan: go test

Reviewers: andybons, Tobias, petermattis

Reviewed By: Tobias, petermattis

Subscribers: petermattis, mrtracy, team

Differential Revision: http://phabricator.cockroachdb.org/D89