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 I was actually looking for this exact thing.  The real issue is activation.  Passivation is actually fine, since the entry would be in the loader/store then (causing a duplicate, but since it is a Set it doens't matter).  However activation could find a miss.