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

- rep:memberRefencesList must be OPV=Copy, otherwise the member-versioning behavior differs
  based on storage implementation.

- update membership docu

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1544748 13f79535-47bb-0310-9956-ffa450edef68