make lifecycle hooks work when using Stash, see #3199

Unfortunately the need to call super.postStop() and friends mandates
that Stash (and FSM had the same problem) extends Actor instead of just
declaring it as a self type. We cannot always win.
Merge pull request #1317 from akka/wip-3199-ActWithStash-∂π

make lifecycle hooks work when using Stash, see #3199