[feature request] Keep track of last successful post to subscribed instances

[mhamzahkhan]

Hello!

I apologise if this is already present. I was thinking it would be useful to store the timestamp of when a relay last accepted a relayed message.

I couldn't find any way to do this from the data stored in Redis, but I might have missed something.

This would be very useful to clean up dead instances that didn't unsubscribe prior to shutting down.