Ability to group data entries

This is a spin-off from http://lists.jboss.org/pipermail/infinispan-dev/2009-December/001968.html.

The requested feature would allow distinct entries to be assigned to custom arbitrary groups. Later in time those groups could be flushed on demand without any knowledge of entry keys.

This feature exists in other open source cache library, see: http://www.opensymphony.com/oscache/api/com/opensymphony/oscache/base/Cache.html