[WFLY-266] Notification support

* Refactor the registry for the notification handlers by splitting the map
  into path segments to mitigate the o(n) cost.
* Rename the registry to NotificationHandlerRegistry and move it to the
  org.jboss.as.controller.registry package (leveraging the
  AtomicFieldUdpated of the package too)

JIRA: https://issues.jboss.org/browse/WFLY-266
I don't think it does any harm, but why this?