=act, rem #2556 Optimize subscriptions of AddressTerminated

* unsubscribe in eventStream is too slow when using many actors
  that are watching remote actors, and becomes a problem for example
  when shutting down such a system
* Previous eventStream solution:
  Stopping 20000 actors took 50355 ms
* This solution:
  Stopping 20000 actors took 764 ms
Merge pull request #2061 from akka/wip-2556-optimize-AddressTerminated-subscriptions-patriknw

=act, rem #2556 Optimize subscriptions of AddressTerminated