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
@ximenesuk: Do you know why that filter's there; perhaps it's a temporary measure required by some pending logging cleanup in server code? Ought I change the filter to at least let `omero.cmd.graphs` stuff through?