Issues in XmppConnection cleanup lead to multiple test failures

Couple of issues in XmppConnection cleanup have been identified when
trying to stabilize bgp_stress_test and bgp_xmpp_deferq_test:

- LifetimeManager is not used correctly - actor is enqueued to the
manager directly from the ManagedDelete method instead of invoking the
LifetimeActor Delete method. Given the right sequence of events this
causes problems when the connection is deleted from both above (due to
test TearDown) and below (due to the mock agent bringing down the session)

- XmppConnections that are rejected during dequeue (because there's
already a connection from the same IP) are not tracked properly. This
causes problems such as deletion of a different incarnation of the
connection (i.e. same IP but not the same data structure).

Both the above result in assertion failures in the LifetimeActor
destructor.