QPID-5795: [Java Broker] Prevent ConnectionAdapter leak when closing a messaging connection

The leak was due to the fact that nothing was telling the virtualhost to unregister the connection child (#unregisterChild)
when the connection was closed.

* Made ConnectionAdapter responsible for causing its own deletion (when the underlying connection is closed).  The call to #deleted() causes the child
 to be unregistered from its parent (preventing the leak)
* Removed the now unnecessary _connectionAdapters map from the VH.  This needlessly duplicated information already held more generally by the ACO.
* Refactored SessionAdapter in sympathy with CA changes.  SessionsAdapters where _not_ being leaked as the session implementation were already telling the model to delete.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1598658 13f79535-47bb-0310-9956-ffa450edef68