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
No, I didn't plan separate pages, the focus on deletion was just as an example. I didn't want to overly intrude on your original documentation in a first PR, especially being unsure for how long we may wish to continue having the server offer the original implementation as a viable fallback. Could indeed target a docs tidy-up for 5.1.0 though in which the previous deletion page becomes somehow subsumed into this one and omits detail particular to the previous, if that seems a good idea.