NotificationBrokerImpl is not scalable

Profiling has shown that NotificationBrokerImpl does not allow for proper scaling. The problem is that each notification incurs a full lock on the listeners map, as it may have been modified by listener registration/unregistration. This leads to severe contention in the notification threads, as they keep bouncing the monitor for no good reason.

We need to bias this structure heavily towards lookups and to do that we need to do the following:

• make the map a constant in lookup path, accessible via an AtomicReference
• rebuild it completely when a registration/unregistration occurs, and update the AtomicReference

There is also a race condition present: notification dispatch is scheduled on an executor, so we can end up delivering notifications after the registration has been closed. Given the change in lookup/update balance, this race window will be enlarged, so fix it by having the task dispatch the event through the registration object, so that unregistration occurs atomically.