OAK-527: permissions (wip)

change permission store to contain a hierarchical view of the entries for a single principal instead of
keeping a flat, unstructured list. this allow to read permission entries lazily or partially depending on
the number and structure of policy nodes and entries in the content.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1489515 13f79535-47bb-0310-9956-ffa450edef68