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. otherwise, I would introduce a bug in getGroup() that only works with "non-optimized" cache store... and of course, I don't wanna test all the cache stores...