Add combiner configuration for min days back filtering

We'd like the ability to have a combiner dedicated to processing old/late events. To do this we want to add a configuration option to set an optional minimum age in days for the events. Any events newer than the min (if set), would be dropped.

Issues with the current diff:
1.  Using an 'int' makes it a bit weird to have 0 mean disabled.  It might be better to switch to an Integer, and have null=disabled.  However, I'm matching current code style for now.
2.  There is no cross field validation for this bean.  Logically min-days needs to be <= max-days, when both min and max are given but there isn't a out of the box way to enforce this.  I can write a new validator annotation, or add some object level check code, but it wasn't clear what we consider best in this case.