Add experimental [Groups] MessageOnlineUsersOnly option for Flotsam XmlRpc groups.

This retrieves and caches information from the PresenceService to only send messages to online users.
This is reported to much improve performance for large groups where most users are offline.
Cache is 20 seconds to balance requests against users not receiving messages until cache updates.
This is an alternative to an approach where login/logout notification is sent directly from simulator to groups service.
However, I'm not convinced that this PresenceService approach is actually better.  Needs more thought.

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