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
https://issues.jboss.org/browse/ISPN-4452
I'm not sure we should use the word "snapshot" here, to me it implies atomicity, but I assume we don't provide that. E.g. when a tx adds two keys to a group, `getGroup()` can return only one of them, not just 0 or 2.