Add high-level pubsub API

Add high-level pubsub API that accomplishes the following goals:

• Hides receive loop from application, possibly by calling application supplied function to handle notifications.
• Uses context.Context to cancel subscriptions.
• Provides way for application to backfill after dialing and subscribing.
• Uses ping to check for live connection and server.
• Optional: support using pool to get connections.
• Optional: subscribe/unsubscribe after running.