OAK-1822: NodeDocument _modified may go back in time

Merged revisions 1594835,1594888,1601838 from trunk to 1.0 branch.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/branches/1.0@1601865 13f79535-47bb-0310-9956-ffa450edef68