always clear ActorCell.sysmsgStash, see #3231

This was not prompted by a concrete failure, just an odd feeling that
proper cleanup might be beneficial even if I currently cannot construct
a case where we would leak something.
Merge pull request #1334 from akka/wip-3231-clear-sysmsgStash-∂π

always clear ActorCell.sysmsgStash, see #3231