MUC: occupant: refactor to allow storing more than just presence for a session Previously a "session" (i.e. a full JID joined to a MUC) was stored as simply a mapping of full JIDs->presence, contained within occupant objects (an occupant object groups all sessions behind a certain nick in the MUC). To enable developing GC3 and other features, it would be helpful if we can store additional metadata when a client joins a room, for example, whether it has opted out of receiving presence stanzas (a GC3 feature). This changes the internal data structure, which shouldn't be used outside this module, it adds a new :get_session() method, and modifies the :each_session() iterator to return the session as an additional result (which should be backwards compatible with code that just consumes the existing two results).

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