clean up DeathWatch semantics, see #3222

- DeathPactException => Stop in defaultStrategy
- ensure that after calling `context unwatch ref` we will not process a
  Terminated(`ref`) anymore, even if it was already enqueued (i.e.
  unwatch() happens between DeathWatchNotification and Terminated)
Merge pull request #1335 from akka/wip-3222-DeathPact-∂π

clean up DeathWatch semantics, see #3222