OAK-945: UserImporter should not cast UserManager to UserManagerImpl

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1511301 13f79535-47bb-0310-9956-ffa450edef68
OAK-938 UserManager does not honour session refresh settings
Tentative fix by wrapping the core user manager into a SessionOperation closure

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1511302 13f79535-47bb-0310-9956-ffa450edef68