Backlog position confusing for new channels on multiple devices

[ccutrer]

when I join a channel on one device, then join it on another, I get no backlog. I would expect to either a) get backlog from when the channel was created, or at the least b) get backlog from the last point of another device. given that it's probably the same logic for when a new device connects at all, and we don't want to send all time to them, maybe the best idea is when a cursor doesn't exist for (device, channel), instead of sending no backlog, send from max(backlog) where channel = ?.