Improved the behavior of instance management regarding

Improved the behavior of instance management regarding
the creation of strong references within the Session;
an object will no longer have an internal reference cycle
created if it's in the transient state or moves into the
detached state - the strong ref is created only when the
object is attached to a Session and is removed when the
object is detached.  This makes it somewhat safer for an
object to have a `__del__()` method, even though this is
not recommended, as relationships with backrefs produce
cycles too.  A warning has been added when a class with
a `__del__()` method is mapped.
[ticket:2708]