Need input from @markfisher and @ghillert but I am thinking that `empty-group-time-to-live` is enough. Not sure the '-on-expiry' adds much value. We should also change the property name to match whatever we decide here (and deprecate `setMinimumTimeoutForEmptyGroups()`.

We should point out a couple of things in the documentation below ...

- Only applies if a MGSR is configured to run
- The actual time-to-live for an empty group will be some time between this value and this value + MGSR.timeout