OAK-482 Group members stored in a rep:members tree

- make all memberOf/membership methods base on lazy iterators
- add abstract class for lazy iteration
- introduce new nodetypes for member references
- add new unit test to verify the structure created by the membership provider
- improve the cyclic membership check
- update docu

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1544665 13f79535-47bb-0310-9956-ffa450edef68