Auto-deletion of old events

[TilBlechschmidt]

Is your feature request related to a problem? Please describe.
When for whatever reason the grid is shutdown with pending events and then restarted at a later point in time, those sessions will be scheduled even though the clients that requested them are long gone.

Describe the solution you'd like
Add a filter to the managers to discard (and terminate) session creation events which are older than a configurable time interval.