document graph traversal reimplementation for developers

Documents the changes to server code brought by https://github.com/openmicroscopy/openmicroscopy/pull/3228; some of the links may remain broken until it is merged. Mostly staged at http://www.openmicroscopy.org/site/support/omero5.1-staging/developers/Server/ObjectGraphs.html.
--no-rebase
@hflynn: Feel free to assign me to that joining once we decide to remove details of the previous implementation.