CPP11 — Valuetypes and arbitrary graphs

Summary: Valuetypes can be used to form arbitrary, potentially circular graphs.
This means that reference counts may never drop to zero and that more
advanced garbage collection is required, which does not come natural to
C++.

An ORB may keep track of circularity by traversing a graph and can detect
if the last outside reference is lost. However, the overhead is significant,
and the solution would be incomplete, as users need not use "proper" refe-
rence counting on graph nodes by ignoring both OBV_* classes and default
reference counting.

Possible solution: restrict CORBA::DefaultValueRefCountBase to
non-circular graphs. Users can decide much better when a graph is safe
to be cleaned up.

Close as duplicate of 2309.