Revision 366204

minor refactor to allow pluggable MessageGroupMap implementations so that we don't have to keep around every single GroupID in RAM if we don't want to (which could very easily become a RAM leak) and can use a provider which uses hashbuckets instead.
added an initial implementation of a hashbucket based MessageGroupMap which should be fine; am waiting on a unit test run to complete before I dare enable it by default :)