OAK-141: issues related to Node.refresh() as opposed to Session.refresh()

Only log a warning for Item.refresh(false) calls as there the current implementation can cause unsaved data to be lost.
For Item.refresh(true) there is no big problem in using Session.refresh(true) instead, as the session could in any case auto-refresh at that point.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1524517 13f79535-47bb-0310-9956-ffa450edef68