Make CORevisionCache non-singleton. The proper way to get a CORevision from a ETUUID is now -[COEditingContext revisionForRevisionUUID:persistentRootUUID:] (maybe it should be moved from COEditingContext+Private.h to the COEditingContext.h header but marked framework private?)

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10133 7b3f36aa-e6db-49c1-8b4c-66eac1997e64