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
I don't think so: It's about how the server does something that requires it to be aware of the data model, and includes useful information for people adding other things to the server that also do model-aware things.