Added test showing group removal not working (esp with longpolling)
- Updated test project to not include private copy of CountDownRange
  and OrderedCountDownRange.
Fixed issues with group token.
- Make sure group token is properly updated on changes
- Made diffset just detect changes in groups.
- Serialize group token even if groups are empty.
- Added tests for the new diffset.

#2040
Call AddEventCore from SetEventTopic.
- This will stop overriden methods from running. DefaultSubscription
  was incorrectly calling AddEventTopic during SetEventTopic.

#2040