OAK-960: Enable session refresh state coordination between multiple session in single thread
Factor refresh logic into its own RefreshManager class

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1517759 13f79535-47bb-0310-9956-ffa450edef68
OAK-301 Document Oak
update section on session synchronisation to reflect the latest changes done in OAK-960

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1517760 13f79535-47bb-0310-9956-ffa450edef68