Add "data-host" to peer roles in chat-db

[drunkplato]

Details

There will be some cases where a ship will be down who is the regular host of a chat. Instead of having it just stop working, we should be able to elect members of our chat as data-hosts who will also mirror the "source of truth" that the host provides. We may need to use solid state subs for this or we can find a mechanism to implement it efficiently ourselves.

[Tenari]

well, each member of a chat keeps their own record of the chat. so the only thing that having the %host be down would actually affect is like invitations, host-only actions like changing the max-expires-at-duration and backlog dumps when new members join.
it's still worth having a mechanism for handling these things, but it's not like chat stops working just because one ship goes down.