OAK-1418: Read performance regression

Delegate time-keeping to a background task

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1575050 13f79535-47bb-0310-9956-ffa450edef68
@trivial add support for unit based time config

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1575104 13f79535-47bb-0310-9956-ffa450edef68