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
Are you sure this synchronized block is needed?  Doing a cursory scan it seems the lookedUpEntries in the various InvocationContext implementations all use a ConcurrentMap.  Or is there some reference I am not seeing.