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
hmmm actually, there is one case where it is needed. when the key is activated (persistence + passivation)
since we iterate over data container and persistence non-atomically, it is possible to a key to be moved from persistence to data container and we don't catch it.