Use a single/less nostr subscription(s) for fetching events

Currently we do `relay.Subscribe` for every request that comes in, we can optimize this by reusing the subscription with updated filters, but that would require us to bring larger code changes to listen to events and dispatch them accordingly (also we are not sure if it is possible to update an existing subscription with new filter changes while not dropping any events on the old subscription)

Not high priority, but we can come to this later if our relay is being affected too much because of concurrent subscriptions