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
Hrmm I would say in the case of a concurrent Creation or Removal that we don't need to care.  This is non deterministic anyways since there is no locks held during retrieval.  I would say for performance only to listen to Activated Cache Entries.