COObjectGraphContext: switch from using NSSet of COObject to track undated/inserted objects, to using NSMutableDictionary of ETUUID->COObject.

Admittedly, the old implementation with NSSet only caused a problem when I was trying something a bit crazy (changing COObject -isEqual semantics in a COObject subclass), but using NSMutableDictionary reduces complexity (there is only one way ETUUID could possibly behave as a dictionary key/set member, whereas it's not so clear for COObject).

git-svn-id: svn://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@9971 7b3f36aa-e6db-49c1-8b4c-66eac1997e64