feat: add exponential backoff to subscriptions

[im-adithya]

• Adds exponential backoff starting from 0s, 2s, 4s ... till 15m and keeps trying every 15m post that.
• Fixes subscriptions not being stopped internally (which can bloat the svc.susbcriptions) when ctx is cancelled during relay connection.
• Also fixes the issue where relays which had connection errors leading to panic on relay.Close() (it was an issue in go-nostr where relay.Connection is not checked before closing)