Marker interface for objects that have known URLs.
If objects that implement this interface are exported, the well-known URLs
are used instead of assigning temporary URLs to objects. In this way, the application
can reduce memory consumption. This also enables objects to have "identities"
that outlive their GC life (for example, instance of the JPA-bound "User" class can
come and go, but they represent the single identity that's pointed by its primary key.)