ISPN-3981 Enhance grouping API to support group manipulation methods

* First implementation. Added getGroup() and removeGroup() methods

This is the base implementation. Added methods to return and remove the keys from a group. This may be extended to support other functionality.

https://issues.jboss.org/browse/ISPN-3981
> yes, but as test cache store, it should respect it IMO.

Fair point, it would be probably beneficial to help find issues instead.