OAK-1161 Simple failover for TarMK-based installations
 - exposed SegmentStore to be usable from the failover code
 - refactored 'get' method so that it can be extended

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1578557 13f79535-47bb-0310-9956-ffa450edef68