RFC: Provide a way to mass update presences about phoenix_pubsub HOT 1 OPEN

Our use case: we use Phoenix.PubSub to broadcast metrics to sockets that push data into charts in our application. To make sure that we don't pull and broadcast metrics that nobody is interested in we are using Tracker. When process subscribes for metrics we are tracking it's presence and check if anyone is subscribed for metric before pulling it from DB.

But we need to persist last event timestamp which we sent to processes so that if metrics server crashes it can recover and send remaining data while it was down. We can use presence metadata to track it but we would need to update lots of presences all the time.

from phoenix_pubsub.