COSQLiteStore: -finalizeDeletionsForPersistentRoot: delete the backing store if it is unreachable (i.e. if you have just deleted the last persistent root that uses that backing store)

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10511 7b3f36aa-e6db-49c1-8b4c-66eac1997e64