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
I use a List to avoid the memory cost of the Set. Having the same keys multiple times it is not a problem (even the values are different) because as soon as the value is inserted in transaction context, it reads return always that value.