OAK-1161 Simple failover for TarMK-based installations
Small changes: 
 - added the ability to unwrap the SegmentStore from the service
  - refactored the URL parts so the HttpStore can be extended easily
   - Journal#setHead now returns true instead of throwing an exception, left in a TODO to not forget about this issue


git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1574120 13f79535-47bb-0310-9956-ffa450edef68