Pension should track state to avoid duplicate notifcations

[johntdyer]

An instance be be scheduled for retirement weeks in advance and if we were to run pension hourly for example that would be 168 duplicate notifications per week about instance retirement. If we kept a statefile of instance ID's we've notified about you could allow for filtering rules, maybe notify once per day or only once total

[erik]

Sounds like a good feature to have!

I won't have time to add this right away but PRs are definitely welcome.