* WARNING: Until both regions upgrade to this revision, prim crossings from regions on this revision to earlier region revisions will not work properly
* e.g. in the attachments cases, the attachments will remain visible but won't be individually detachable
* This change may seem to have more costs than benefits, but I'm doing it because I can soon reuse this changed existing code in another context - it seems better not to
proliferate similar but slightly different xml serializations.
* I think this has been done cleanly from inspection and testing, but if prim creation or load suddenly starts playing up more than usual, please open a mantis
* This also has the effect of stopping the archiver generating ghost in-world prims
* Some code dupliction also removed
* Adding to a scene is now parameterized such that one can choose not to actually persist that group
* This is to support a use case where a module wants a scene which consists of both objects which are persisted, and ones which are just temporary for the lifetime of that server instance
