Use local sender variable rather than static property

[jarlrasm]

I am not sure if using Sender rather than sender here was intentional, but it looks wrong. We believe it is currently a possible reason for a bug we have where a second message, received when the PersitentActor is rebooted, don't get the expected reply back to its sender.

[Aaronontheweb]

thanks @jarlrasm - I'll review this soon