OAK-1295 - Recovery for missing _lastRev updates (WIP)

Refactor
-- to use enum for string constant
-- clock handling moved to single method
-- Strongly type getters for property

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1583892 13f79535-47bb-0310-9956-ffa450edef68