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
This sentence should actually be the main Encouragement. Unfortunately, it is itself very hard to read, as it is far too long and has plenty of negations (``no test failures...``) - maybe try to simplify ?