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
We should mention the transaction semantics of removeGroup since it does this removal in a new transaction.

Also I worry that this is a prone to having lock timeouts if the current transaction on this thread holds one of the locks for a key in the group.