model: Investigate alternatives of passing messages to `RecentSenders`

[sm-sayedi]

This is a follow-up to #692.

Quoting Greg from #692 comment:

Ideally we'd have MessageStore.reconcileMessages handle passing the messages on to RecentSenders. That way as we add more and more data structures that similarly want to know about fetched messages, there's just one place we have to wire them in.

Now that we have a central message store (#648), we may want the RecentSenders data structure to receive its data (messages) from MessageStore.